While it's absolutely true that you don't need a big traumatic event to cause mental illness and those little things can add up, it still doesn't mean that you need an external stimulus to trigger it.

Using my situation again because it's one I'm deeply familiar with, when I started to see my psychiatrist, I actually found out that my outbursts and other bad behaviors from when I was a toddler, were likely early manifestations of my depression. Because it literally started before I was fully aware of my surroundings, and the fact that my mom and both siblings have long term depression, my shrink said it's likely actually inherited and not just an inherited susceptibility.

Now granted, I have no idea if that's common, but at least in one case, it really does seem to be a genetic cause instead of a genetic predisposition. And I think the biggest contributors to the "susceptibility vs causative" debate are that there are many kinds of depression, it's likely not monogenetically inherited, and they don't know all the genes that could contribute. This basically makes a big black box where geneticists and physicians try to correlate mutations that we can find, with symptoms that vary person to person, without knowing all the genes that may be involved. And if the study is designed REALLY poorly, they'll lump everyone who's had a depressive episode together to compare against the control group, despite the different classifications.

Sorry for the wall of text, it's just something I've been trying to learn more about the last few weeks. On a side note, I hope you're doing alright. Depression and anxiety are seriously awful.