The Prophet Muhammad (S) has said, “O' ‘Ali! Associating with three (groups) of people kills the heart: low people, rich people and (frivolous) talk with women.”
The points which we can benefit from in this tradition which is an excerpt from the testament of the Noble Prophet (S) addressed to the Commander of the Faithful, ‘Ali b. Abi Talib (as) include the following:
1. We can draw a conclusion in regards to the life and death of the heart.
In this tradition, the word heart has been used as an allusion to the spirit and soul of the person. In addition to the physical body having the states of life and death, the spiritual heart and soul also have both life and death and this has been referred to as the “spiritual life and death”.

إِلٌـهِـي أَلْبَسَـتْـنِي الْخَطَاياَ ثَوْبَ مَذَلَّتِـي، وَجَلَّلَنِي التَّبَاعُدُ مِنْكَ لِبَاسَ مَسْكَنَتِي، وَأَمَاتَ قَلْبِي عَظِيمُ جِـنَايَـتِي…

“O' Allah! Surely my many errors have covered over my true essence and my spiritual distance from You has clothed me in the separation from You and my major crimes and sins have killed my heart…”[1]
This supplication clearly proves the point mentioned in the tradition that the heart too possesses the states of life and death and in principle, this whispered prayer of those who turn to Allah in repentance is proof of the reasons of the death and life of the heart.
In this prayer, the Imam is in essence saying, “O' Allah! Enliven my heart through turning back to You in repentance!” Thus, this means that it is through turning back in repentance to Allah (SwT) that give us life while sinning against Allah (SwT) is what causes our true death!
There are some people who have thousands of things happen to them in their life that should act as lessons for them. In addition, there are others who hear thousands of verses of the Qur'an and traditions which contain points of admonition, however these things do not have even the slightest effect on them and they do not waver in the least since their hearts have died.
As we know, there are two ways that we normally judge life: one is through the feelings and emotions and the other is the physical movement. Therefore, if a creature has no feelings or emotions or is an entity which is not moving, it is likely that that creature is dead. This can be seen in the Qur'an in which in its detailed discussions on the spiritual life and death that it, for example, speaks about some 'dead' people as actually being at the level of those who are alive when it states:

وَلاَ تَحْسَبَنَّ الَّذِينَ قُتِلُوا فِي سَـبِيلِ اللٌّهِ أَمْواتاً بَلْ أَحْـيَآءٌ عِنْدَ رَبِّهِمْ يُرْزَقُونَ

“And do not consider those people who have been killed in the way of Allah as being dead, rather, they are alive in the presence of their Lord receiving sustenance.”[2]
In another place in the Qur'an, a group of people who are (apparently) alive are classified as being dead when we are told:

إِنَّكَ لاَ تَسْمَعُ الْمَوْتَى وَلاَ تَسْمَعُ الصُّمِّ الدُّعَآءِ

“And surely you can not make the dead hear and you can not make the deaf hear (your call).”[3]
In another place in the Qur`an we are told:

لِيُنْذِرَ مَنْ كَانَ حَيّاً

“So that you may warn those who are (spiritually) alive…”[4]
Thus, those who receive admonition from the Qur`an (and act according to it) are considered alive in the view of the Qur`an.

The Life and Death of the Hearts
Humanity Possess many Examples of Life and Death!
1. The first example of life and death is seen in the plants in the form of their growing and development, their nourishment and their reproduction - and all plants are the same in these regards.
2. Another form of life and death is seen in the animal world. This too is a clear example of the feelings, emotions, movement and action (of a life form) and in these regards, all animals are the same.
3. However, the third form of life and death which is limited to humanity and is what separates them from plants and animals is the life which can be divided into “physical” and “spiritual” life.
This is what which has been mentioned in the Islamic traditions and is referred to as the “spiritually alive heart” in which the meaning of heart in this definition is the soul, intellect and emotional capacity of the person.
In the words of the Commander of the Faithful, ‘Ali b. Abi Talib (as) in his speeches and short sayings narrated in Nahj al-Balagha, we see a great emphasis placed on this issue. In speech 110, he says the following in regards to the Qur`an:

تَفَقَّهُوا فِيهِ فَإِنَّهُ رَبِيعُ الْقُلُوبِ

“Ponder upon it (the Qur'an) since this is the spring of the hearts (it enlivens the hearts just as the spring season brings the trees to life).”
In speech 133, in regards to wisdom and knowledge, he has stated:

هِيَ حَياةٌ لِلْقَلْبِ الْمَيِّتِ

”(Wisdom and knowledge) are life for the dead heart.”
At other times, he places the sickness of the heart at the same level as the sickness of the body. For example, in short saying 388, he has stated:

وَأََشَدُّ مِنْ مَرَضِ الْبَدَنِ مَرَضُ الْقَلْبِ

“Even worse than the sickness of the body is the sickness of the (spiritual) heart.”
In addition, in short saying 349, he has stated:

مَنْ قَلَّ وَرَعُهُ مَاتَ قَلْبُهُ

“The one whose level of precaution (Wara’ - a level higher than that of Taqwa) is low, his heart is dead.”
There are many more such phrases used by the Imam in this regards.
From another point of view, the Noble Qur'an believes in a particular form of seeing, hearing, understanding and comprehending other than the apparent understanding of these powers just as it is said in regards to the open disbelievers:

صُمٌّ بُكْمٌ عُمْـيٌ فَهُمْ لاَ يَعْقِلُونَ

“Deaf, dumb, and blind, they do not understand anything.”[5]
In another place in the Qur'an, the hypocrites are referred to as having a sick heart and since they do not want to change, Allah (SwT) adds to their sickness:

فِي قُلُوبِهِمْ مَرَضٌ فَزادَهُمُ اللٌّهُ مَرَضاً

“In their hearts is a disease (and due to their obstinacy) Allah has added to their sickness.”[6]
Those who do not have the fear of Allah (SwT) within themselves are introduced as people possessing hearts of stone - rather, their hearts are even harder and firmer than stones:

ثُمَّ قَسَتْ قُلُوبُكُمْ فَهِيَ كَالْحِجَارَةِ أَوْ أَشَّدُّ قَسْوَةٍ

“Then your hearts hardened after that, so that they were like rocks, rather worse in hardness;”[7]
In relation to a group of disbelievers, there is a phrase that is used in the Qur'an in which they have been referred to as being impure:

أُولٌئِكَ الَّذِينَ لَمْ يَرِدِ اللٌّهُ أَنْ يُطَهِّرَ قُلُوبُهُمْ

“Surely those are the people whose hearts are not purified by Allah.”[8]
In another place in the Qur'an we read that the only ones who shall hear and respond to the call of the Prophet (S) are those who are spiritually alive and possess the ears to hear such a call:

إِنَّمَا يَسْتجِيبُ الَّذِينَ يَسْمَعُونَ وَالْمَوْتى يَبْعَثَهُمُ اللٌّهُ ثُمَّ إِلَيْهِ يُرْجَعوُنِ

“The only people who shall accept and answer (your call) are the people who (spiritually) hear you and as for the (spiritually) dead, Allah will raise them, then to Him they shall be returned.”[9]
From these different phrases and indeed many other similar phrases in the Qur`an, we can clearly see the angle which the Qur`an takes on life and death and that it is the humanistic and logical angle of the discussion since all of the worth of a person is contained and based on this aspect of his life (his spiritual traits and worth and not his physical existence).
The factors that determine a spiritual death and life are very many, however undoubtably things such as hypocrisy, pride, prejudice and nationalism, ignorance and the performance of the major sins are things which kill the heart.
Thus, going back to the tradition we started out with, the Noble Prophet (S) has stated, “Sitting and associating with three groups of people kill the heart: sitting with people who themselves are polluted with sins kills the heart and by a person interacting with such people, he will spiritually descend to such a level that he himself shall become an instrument through which others will be able to learn a lesson from (in how not to act).”
The word used in this tradition for “people who themselves are polluted with sins” is “نذل” which means a low or despicable individual from the point of view of his morals and etiquette, faith, thoughts, etc…
There are many ways in which a person can acquire status and character, of which there are four principle ways:
1. The training and upbringing from the mother and father;
2. The society and his interactions with the people and which is common within the society and environment;
3. His teachers at school;
4. His friends and acquaintances, and this is, perhaps the most influential of all ways which affect a person's status and character. Thus, if one's friends are corrupt, then the one who associates with them will also become corrupt. However if they have upright morals, then the person who is around them will be the same.
Within a person, there is a psychological trait of “involuntary following” in which anything a person sees others do, he would also automatically perform. For example, if a person was to yawn then almost instantly, others who are in the same vicinity would also inadvertently begin to yawn!
Many of the actions which people perform are the same and one of these is the effects of the surroundings which a person is in and the effects of the morals and etiquette which friends have upon one another.
It has been stated that if you see a person who has an ego however is reserved, and if (due to his being reserved) you are not able to determine his morals and character in your first and second encounter with him, then you must look towards his friends and those who associate with him to guage his character:

فَانْظُرُوا إِلـى خُلَفَائِهِ وَ جُلَسَائِهِ

“Look towards those who are his friends (lit. followers) and those who associate with him.”[10]
If you see that a group of spiritually low people are always gathered around him, then do not be tricked and fooled by seeing his heavily used and worn out prayer mat, since:

إِنَّ الْمَرْءَ عَلى دِينِ خَلِيلِهِ وَ جَلِيلِهِ

“A person follows the faith of his friend and those who are in his company.”[11]
Therefore, sitting with those people who have negative ethical traits kills the heart.
2. Sitting with rich people also kills the heart of a person. Of course, the meaning of rich people is a specific type of rich person. This tradition does not mean that any person who happens to be rich is bad since, how many times have we seen that sitting with rich people actually increase our prosperity and happiness due to their character!
It is mentioned that Prophet Ibrahim (as) was a very rich person, however when an individual remembered Allah (SwT) and said:

سُبُّوحٌ وَ قُدُّوسٌ…

he gave half of his wealth to that person for the sake of Allah (SwT)!
Thus, the meaning of the tradition under review and the mentioning of rich people refers to something else. These words refer to those who are not devoted to the faith. Rather, they are devoted to their money!
However since usually a majority of those who are extremely affluent do not face any difficulties in their life (due to their wealth), are always pleased with themselves (at their enormous amounts of wealth and their status), show self-importance and have drowned themselves in the vanity and pride, it can be seen that when a person sits in their company or speaks to them, their discussion would always be in regards to their income or expenses and you never hear anything from them about Allah (SwT), the Prophet (S) and any other spiritual issues or very little…
When such people speak, one may observe that they could easily list seventy different types of food which they have eaten! Their knowledge and facts on the various types of food, nonsensical things and instruments of fun and pleasure of this world is great, however when a discussion on ethics and morality comes up, these same people would remain quiet - it is as if the dirt of the grave has covered them!
Thus, it is due to gatherings and sitting with rich people who possess these traits that kills a person's heart.[12]
3. Speaking with women also kills the heart. Of course this does not refer to women of true faith who follow the teachings of the religion. Rather, this refers to all other women since a majority of the discussion which (other) women indulge in are in regards to their own personal beautification and the beauties of the world. More often than not, their discussions are in regards to material pleasures or in picking the defects and faults of others.
In summary, any time a discussion takes place in regards to the transient world and the ornaments, beauties and beatifications, such talks will always lead to the spiritual death of the heart. Just as we know the word “tradition” (hadith) refers to speaking and this necessitates the act of gathering together to discuss things.
The Qur`an brings forth a very interesting phrase in relation to certain women and says:

أَوَ مَنْ يُنْشِيءُ فِــي الْحِلْيَةِ وَهُوَ فِي الْخِصَامِ غَيْرُ مُبِينٍ

“Is the one who has been brought up among trinkets (of beautification) and is inconspicuous in contests (should be associated with Allah)?”[13]
This is the case since these sorts of women are brought up with an attachment to the adornments of the world - the fabric of their thinking is the fabric of beautification and that is all.
An important point which must be mentioned is that the things which lead to the life and death of the heart are numerous and thus, a person must be careful that he does not relegate his soul and essence to just anyone.
Without doubt, a person acquires a good part of his thoughts and ethical traits from his friends. From the logic of Islam, this form of influence (one's friends) is at such a level that in the taditions which have come to us from the Prophet Sulayman (as), he has stated:

لاَ تَحْكُمُوا عَلى رَجُلٍ بَشَيْءٍ حَتَّى تَنْظُرُوا إِلَى مَنْ يُصَاحِبَ. فَإِنَّمَا يَعْرِفُ الرَّجُلَ بِأَشْكَالِهِ وَأَقْرَانِ، وَيَنْسِبُ إِلـى أَصْحَابِهِ وَأَخَذَنْهِ

“Do not judge a person until you see whom he associates with since a person can be known by the friends and associates he keeps with and he is affiliated to his companions and friends.”
The Commander of the Faithful, ‘Ali b. Abi Talib (as) has stated:

وَمَنْ إِشْتَبَهَ عَلَيْكُمْ أَمْرَهُ، وَ لَمْ تَعْرِفُوا دِينَـهُ، فَانْظُرُوا إِلـى خُلاَفَائِهِ. فَإِنْ كَانُوا أَهْلَ دِينِ اللٌّهِ فَهُوَ عَلى دِينِ اللٌّهِ، وَ إِنْ كَانُوا عَلى غَيْرِ دِينِ اللٌّهِ فَلاَحَظَ لَهُ مِنْ دِينِ اللٌّهِ

“Anytime the state of a person is not known to you and you do not know his faith, look towards his friends. If they are people of the faith of Allah then he too shall be on the faith of Allah, and if they are on other than the faith of Allah then he too shall not be on the faith of Allah.”
Without a doubt, the role that a friend plays in the felicity and adversity of a person is more important than any other thing. Such a friend will drag a person to the ultimate level of spiritual destruction and annihilation - however it is also possible that he may pull him towards the loftiest levels of greatness and spiritual worth!

Notes:
[1] The Whispered Prayers of Those Who Turn in Repentance, as-sahifatul Kamilatul Sajjadiyah, Prayer 1
[2] Surat ale Imran (3), verse 169
[3] Surat al-Naml (27), verse 80
[4] Surat Yasin (36), verse 70
[5] Surat al-Baqarah (2), verse 171
[6] Ibid., verse 10
[7] Ibid., verse 74
[8] Surat al-Maidah (5), verse 41
[9] Surat al-Anam (6), verse 37
[10] Bihar al-Anwar, vol. 74, pg. 197
[11] Ibid.
[12] It is possible that one of the reasons why associating with rich people leads to the spiritual death of a person's heart is that the source of a majority of the corruption and perversion within the society also stems from the rich people who are completely unaware of Allah (SwT) and are completely drowned in the vanities, glories, fun and pleasure of this transient world!
Most often, any voice of reason which would lead to their moral refinement, return of human qualities and ethical traits seems strange to their ears! It is for this reason that such people always stood in the first line against the Prophets of Allah (SwT) and their message, which the rich always assumed was against them directly, was one of justice and equality and the assistance of those who are downtrodden. (Tafsir-e-Namuna, vol. 12, pg. 60)
[13] Surat al-Zukhruf (43), verse 18