There's more than a few clues that at least some depressions, or rather - depression like states - on short timescales are closely related to our brains dynamic "electrochemical" state than of anything else, as disrupting the dynamics in different ways appear to - sometimes - profoundly affect depressed states.

On longer timescales it seems even more complex. Similarly to ECT, some people revert to a depressive state somewhat quickly after being affected by various drugs, while others do not.

I sometimes think that when we start understanding depression, we'll have to reevaluate our perspectives on both ourselves, society, and humanity as such much more than most are comfortable with.

Not in any mysterious sense, but more in the sense that depression on an individual level might very well be a symptom of society level issues, as well as it is an extremely debilitating psychiatric condition.

While there are markers for depression in individuals, I can't escape the feeling that to some extent, it's nothing but a convoluted way to encode tolerance levels to what is in reality a "toxic" surrounding. Exactly what the toxicity could be is not at all clear, but I don't expect it'll have a high compatibility with eg economic interests in any way, shape, or form.