What I would say depression is is the brain invents an image of "what should be" (in the "future") - which is "the self". For example "I will be a successful entrepreneur with an attractive partner and 2 or 3 kids, will have a house in a warm green area", or something like that and at some point life says, no you can't have those, that image you've built is false, bro.. One falls ill or the wife leaves or a kid dies and all the startup companies go bankrupt. So that precious image of "what should be" that the brain has built does not reflect reality, there is a conflict in the brain between those two, and the structure of "what should be" in the brain means a strong attachment/ it was a source of pleasure for thought. But the image HAS to correlate with Reality (otherwise one goes completely insane), so the Thymic and Autonomic aspects of the META get activated, intense sorrow, anger, loss, apathy, loss of energy.. That may be what is diagnosed as "depression". But perhaps it does not necessarily needs to be escaped from, "cured" or medicated right away. Perhaps one should instead observe the whole movement of thought that is actually happening, without any choice, control, effort or opposition. Perhaps there simply should be an observation that completely correlates with Reality, reflects Reality. The old structure of the self has to dissolve and that alone would naturally and inevitably bring peace and order.

The modern therapies of depression may often completely miss the point. They offer various escapes and fast medication without understanding what the central issue seems to be.
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When thought gets dissatisfied in the pursuits of the "material world", it often starts to chase so called "spiritual" goals, like finding "God" or "nirvana" or something along these lines. This seems to be a completely different direction than the previous one but in fact it is the same movement of the self as before only seeking different kind of experiences, pleasures and sensations, and ultimately different kind of illusions.