I disagree with this argument. I think another one's more salient. For a pack animal such as primal humans probably were, a certain type of moderate depression is a survival mechanism for dealing with low social status. Your appetite and libido decrease, you become inassertive, you accept mistreatment from others. Subjectively, this process and experience sucks, but lacking desire to mate with the alpha female keeps the omega male alive. Much more importantly (since nature doesn't care much about unsuccessful individuals) it's a survival mechanism for the pack; the depressive response ensures that, in extreme scarcity, the less successful individuals slink away and accept starvation, rather than challenging and potentially causing harm to the more important ones.

In modern society, this reaction is completely useless and utterly maladaptive. It also has a tendency to fire in people at all levels of social status, because modern society (with its dynamic, subtle, and multileveled play of social status) is confusing to the primal mind. So a depressive response can be rightly considered an illness.

Civilization expects everyone to assume the role most closely analogous to high beta/low alpha. People who have omega traits tend to be depressives; people who have too many alpha traits are sociopaths.