It's a moot point - depression does seem to correlate with some sort of chemical imbalance, but is that cause or effect, or both? Blocking the reuptake of serotonin/dopamine pharmaceutically is essentially tweaking part of a complex machine that is only understood in a limited way, but does seem to have some affect, though variable depending on the individual.

Experiences and how they program the links between memories and their association with emotional response and perspective seem to also play a key role, something which talk therapy and things like EMDR seem designed to try and address.

To me, it seems like the human (and likely the brains of any creatures considered sentient) have evolved to a point of complexity, where they are capable of setting & revising their emotional state/perspective drawn from memories, but with the side-effect of sometimes creating perspectives that are upsetting & traumatic to deal with.

In short, we're an unfinished & still evolving piece of work.