I had a difficult childhood, can identify my depression starting back then, did well in school but never really had friends (and still don't). I'm not sure how much of my long term depression is down to the things that happened back in childhood, but whenever I've had therapy in the past they've always wanted to concentrate on those childhood issues, even when I've had more pressing immediate life issues to deal with. The causes of depression are complex and for me they probably include a genetic predisposition, a difficult childhood and some idiosyncratic personality traits that are hard to pinpoint.