In the name of Allah, Most Gracious, Most Merciful.
Allah the Exalted, while promising that the retribution of good and evil will happen on the Day of Judgment, in fact, however, sends a small sample of that retribution here, at every instance and at every moment. If man discovers joy in his heart, he is being rewarded for making someone happy; if he is sad, it is that he has made someone sad. These rewards are presents from the other world and signs of the Day of Retribution, so that we may understand, with so little that which is great. We are being shown from a large warehouse, a small handful of wheat.
So that you know and remember that all discomfort, all darkness, all depressions that befall you, are the bitter fruit of sin and the pain you yourself have caused, even though you do not remember the evil that you have done through negligence, or ignorance, or because of a bad companion with no Iman (faith) who downplayed the sins in your eyes, to the point where you no longer considered them sins. Think of the reward: what is the level of your fulfillment and what is the degree of your depression? Certainly, depression is the punishment of sin and fulfillment is the reward of submission to Allah.