IDK how I became depressed. I believe I was born with it and then my addiction to (or better yet, my stop abusing of) heroine, triggered the major issues going on to this day. My mother suffers from depression. However, I suffer from Biopolar disorder with episodes of major depression... which occur way too often, even on medication. After years of therapy, I'm still no closer in knowing what the trigger is to these episodes.
I don't think it helps that I can't remember much before I was 7 (I think I blocked those years out) or that there was sexual abuse when I was younger... but we're not going into that....