What is natural, the thinking goes, is best. If we are designed to suffer depression in response to life’s ills, there must be a good reason for it, and we should allow it to take its painful and natural course.
The evolutionary explanation for depression is rooted in “Group Rank Theory“, which posits that a lot of our social intuitions were formed to help us establish pecking orders within our tribes, which promotes social cohesion.
From that, it’s often necessary for individuals within a group to give up rank. Under normal circumstances giving up rank is fought vigorously, but depression gives individuals a rationale for doing so without resorting to violence. Mania serves a similar purpose, but for opposite movements within a social order.
If that’s true, I still don’t think that’s a good justification for not treating depression. I tend to abide by the “live natural” creed, but mostly because living natural aligns our instincts and desires with beneficial outcomes.
If that link is severed, then other alternatives can be explored.