Well, perhaps it happens because of our ambition and high expectation. An ambition to be rich, or to be smart, or to be famous will push you until you get stressed and depressed. Surely we could achieve it but it will take a time. But we usually are not patient enough to enjoy the process of our struggle. 

I think that is the reason why we always get depressed.

i dont know why people like to add NOWDAYS before every problem.

problems have been on earth since mankind started its existance. depression is no exception.

I think people don't get enough sunlight. Many people are indoors all day.

I figure the cause of depression varies a lot and it is hard to generalize depression as one single factor.

All people feel depressed from time to time , but clinical depression is an ongoing debilitating disorder that interferes with everyday life of the person. Not all experts agree on what is "normal" sadness and what is " disorder " . But certainly some people have deep negative feelings , sometimes accompanied by excessive feelings of guilt and worthlessness . You can not always control circumstances , and bad things end up affecting you . But you can take practical steps to be dominated by negative feelings. We need to seek God , if I can advise you, this is my advice : Do not bring the past to the present ; look forward. Reconciled with God so that his prayer is heard ; be sure to practice physical exercises. Get involved and take responsibility for the house of the Lord . Stand firm , do not be discouraged because you will win this war.