For every one of these anecdotes, there's another where the kid has a great childhood and school experience but still ends up depressed. Or one that is severely bullied but ended up enlightened and happy. Psychiatrists can come up with whatever post hoc explanations they want, but as you said, it's multifactorial, and therefore does not admit simple explanations.
Anyone can succumb to depression. I was giving my experience as an example to show that it’s important for people to understand why they are depressed, and that psychiatry may help you find the reason.
Yeah, I'd fit in that camp. I don't think I've got any repressed childhood trauma hiding under 20 layers, ready for a psychiatrist to uncover, I'm just sad. There are good days and bad.