I found this article on a possible evolutionary cause behind depression very interesting: Depressed To Impress | Pop Psychology
Hm ... I'd be more comfortable calling it an evolutionary adaptation to depression rather than a cause, but I guess I'd have to read about it more.
I don't like how the wording makes it seem like people intentionally decide to be depressed in order to manipulate others. I know that's not what the author was trying to say... but still, it comes across that way.