Genetics probably, but right now I'm not too sure.
Sometimes I don't even think that I have depression and am trying to excuse or justify personal inadequacy by pretending I'm ill or that my depression is a punishment for being an unpleasant and useless human being.
These are all fairly typical beliefs in depression so perhaps depression is triggering my depression. I'm pretty depressed right now and really struggling to comprehend that depression is an illness and not a punishment. In my clearer headed moments I recognise that depression has occurred in several generations of my family so genetics has a part to play.
When I was younger, therapists wanted me to explore the way my dad abandoned my family. Until then I had never considered that a trigger, mainly because I was relieved he wasn't in our lives anymore as he had made everyone so unhappy when he was at home. Therapists revisting this theme time and time again actually made some depressive episodes much worse. I was so frustrated they wouldn't explore other issues that were far more important to me and because they were so unrelenting in their pusuit of their agenda that, eventually, I just capitulated to get them off my case.
So I'm inclined to say that genetics and inappropriate therapies have made me depressed.