I think that depression is as diverse as the people who have to live with it.
For some, it's predispositioned.
For others, i think it's simply a backlogue of mismanaged coping techniques that cause your brain to react a certain way... it's almost like a trained thing.
For others still, it's a physical reaction to the environment. IE: SADS or other depression related to weather.
and then, for others, it can be a combination of all of it. It's a very complicated disorder, and that's probably why it's so hard to treat.