Depression can come from a chemical imbalance in the brain, while that is an oversimplification and someone could be suffering from shit life syndrome, which would be self-explanatory, life isn't as simple as you have these conditions the person is not depressed.

The terrifying truth of it is, your brain sometimes turns against you making a difficult situation ever more difficult. Why do people get depressed is not at all an easy question.

It would also be a very difficult thing to study. For example you could find a correlation between drinking heavily and depression, but how would you ever know if depressed people are more likely to pick up the bottle, or if alcoholism leads to depression?


As for the person that slumped and died in the street, without any more context who knows? He could be a generally happy person that died all of a sudden.