1. Sometimes, distress and anxiety are related to a dark and uncertain future which lie ahead of a person and manifest itself to him. The possibility of blessings leaving a person, falling into the grip of the enemies, weakness, sickness, inability to work, desperation and his need of others all result in a person being put through mental trauma.
However, true faith in Allah (SwT) the All-Able and High, the Merciful and Compassionate, the Creator who has taken it upon Himself to be sufficient for all of His servants is able to remove all forms of worry and anxiety and grant a person tranquility since we are helpless in regards to the future events, however Allah has the power, ability and mercy (to help us).
2. Sometimes the bleak past would make an individual busy within himself and this would cause him anxiety. Showing grief over sins that were committed in the past, feeling remorse from one's inabilities and past slips would cause a person worry.
However, paying attention to Allah (SwT) the All-Forgiving, the One who accepts repentance and is the Merciful would grant tranquility to a person!
In reality, these traits of Allah (SwT) would convey the following thoughts to the person that, “Take your appeals of culpability to Him, seek forgiveness for your past acts and make amends for what you did in the past since He is the Forgiving and the One who will make accept the amends for your misdeeds.”
3. Weakness and an inability in the face of natural occurrences and sometimes even in the face of enemies - both internal and external, makes a person upset and insecure such that he may think to himself, “What am I able to do in the war against all of these powerful opponents? What can I do in the face of other challenges and struggles?”
However when one remembers Allah (SwT) and begins to rely upon His power and mercy - the Power which is the greatest of all powers such that there is not a single other thing which has the potential to stand up to Him, then the person's heart would become at ease and he would say to himself, “Yes, I am not alone. I am under the shadow of the Mercy and Assistance of Allah (SwT) - the never ending Source of Power.”
4. Sometimes the source of grief, insecurity and that which pains a person is his feeling that his life is worthless and that the goals which he has set for himself are insignificant.
However, the one who has true faith in Allah (SwT) and has accepted that traversing the path of perfection is the greatest goal and knows everything which transpires in life must be experienced while on the path of perfection, would not feel that his life is worthless, that he is without goals, nor that he is an uncertain wanderer, filled with distress.
5. Another reason for grief and insecurity is that sometimes in order for one to reach his a goal, a person may need to bear difficulties. However at the same time, he does not see a single person who encourages or appreciates the troubles and difficulties which he has gone through. This feeling of ingratitude would cause him great stress and would cause him to sink into a deeper state of anguish and insecurity.
However when he senses that other people are aware of his struggle, value his efforts and are ready to reward him, then there would not be any room for grief and insecurity.
6. Having bad thoughts of others and empty delusions is another reason which leads to insecurity and grief and this is one of the things which a large number of people see in their life. However, how is it possible for us to deny that focusing our attention on Allah (SwT) and His never ending grace, His commandment to us of maintaining good thoughts and suppositions of other people - which is the responsibility for every single person who claims to possess true faith, would not remove this state of grief and trepidation which is inside a person and would cause him to have tranquility and certainty in his life!?
7. Worship of the transient world and infatuation with the gold and glitter of the life of the material world we are living in is one of the greatest reasons why a person faces distress and insecurity in his life.
This point is apparent to such an extent that sometimes if a person is not able to find a particular color of clothing or a color of shoes, hat or some other item of apparel in the shopping centres or any other item which he uses in his daily life, that the thoughts of such a person who worships this transient world may become so engulfed and enraptured that he would spend hours, days or even weeks in a state of insecurity and anxiousness until he can find what he wants!
However, true faith in Allah (SwT) and constant focus on the spiritual freedom which a true believer possesses - in which he is constantly enraptured in austerity and devotion to Allah (SwT) and is not bound by the clutches of the gold and glitter which accompany the material world, would bring an end to such feelings of insecurity and anxiety.
When the soul of a person reaches to that level of spiritual expanse that just like ‘Ali b. Abi Talib (as) can say:

إِنَّ دُنْـيَاكُمْ لاََهْوَنُ عِنْدِي مِنْ وَرَقَةِ فِي فَمِ جَرَادَةِ تَقْضَمُهَا

“Surely your world has as much value to me as the leaf of a tree which is in the mouth of a locust who is chewing upon it.”[1]
then at that point how is it possible that having grief or fear due to the material world or losing it from one's grip would cause one's soul to be affected and to cause a sense of grief and insecurity in his heart and mind!?
8. Another important factor which leads to grief and insecurity is the fear of death as this is something which always causes a person's soul trouble. Since death is something which can strike a person at any age especially at the times of sickness, wars, insecurity in a country and other such things, thus this sense of insecurity and grief is a general trait and is seen by all people.
However, in the Islamic world-outlook, if death meant annihilation, inexistence and the end of all things - just as the materialists of the world believe, then this grief and anxiety would be completely in place for us to experience! In addition, it would be completely inline for a Muslim to be frightened of such a death which is the end of all things and brings closure of all of his desires and wishes.
However, anytime a person comes under the shade of true faith in Allah (SwT), then at this point, he would consider death as being a bridge to a wider, more spacious, and spiritually higher world. Passing through the stage of “death” would be just as passing from the jail cell into the free society.
At this point, there would be no meaning to grief and anxiety! Rather, when such a death comes while on the path of fulfilling one's responsibilities, then at this time, death is something that is liked and anticipated!
Of course, the reasons for grief and anxiety are not limited to only these. Rather, we can list many more reasons why people feel grief and stress in their lives. However we must accept the fact that a majority of the cause of this grief and anxiety are related to one of the eight reasons listed above.
When we see the reasons for anxiety actually melting away and being removed when one brings about true faith in Allah (SwT), then we will definitely confirm the fact that the remembrance of Allah (SwT) is that which brings tranquility to the hearts:

أَلاَبِذِكْرِاللهِ تَطْمَئِنُّ الْقُلُوبِ

“Now surely with the remembrance of Allah are the hearts put at ease.” !

Note:
[1] Bihar al-Anwar, vol. 75, pg. 359