You can split definitions of depression in a number of ways, but one that's been around for a long time is exogenous (external causes) and endogenous (internal causes). The idea that symptoms are only due to life situations is simplistic and wrong.
This feels a bit like the straw man arguments about economics only treating people as rational actors, whereas economists have been aware of the issues with that for decades.
You may be an exceptional character: very dumb or smart; maybe too sensitive of too unemphatic; on some strange neurodiversity spectrum; maybe you was raised by a bit too personal parents; maybe you adopted radical or odd ideologies etc., ad infinitum
Or you may live in an exceptional external situation. It can be good (you are a rock star) or bad (many).
Either way, either case or a bad mix of internal and external may wear you out in the long run, and you stop functioning, you burn out, you become depressed.
This feels a bit like the straw man arguments about economics only treating people as rational actors, whereas economists have been aware of the issues with that for decades.