Every second person complains of stress, anxiety and depression. There are three main causes for this:
1. We have shifted our focus from Allah Ta`ala towards our abilities and resources, and towards people around us. Thus, when everything around us fails us, we lose hope in life and sink into a state of  depression. We have forgotten that our Allah Ta`ala alone has placed us in that condition and our Allah Ta`ala alone will remove us from that condition. Our attention and focus should constantly be directed towards the unlimited treasures of Allah Ta`ala and not towards our endless weakness.
2. We worry and concern ourselves over such things that are beyond our reach, and neither are they in our control. We become overwhelmed thinking about matters that have already been decided by our Allah Ta`ala. Mortal man, however, fails to accept that he has absolutely no say in such issues. Hence we become so involved in matters and things that are beyond our control that we end up neglecting what is in our control. The end result is disorder and helplessness which leads to depression.
3. Sins also lead to depression. It is impossible to be happy if Allah Ta`ala is unhappy with us. Our evil actions are the fuel to the fire of destruction and depression. Righteous actions will fill the heart with happiness.