I never got depressed until I wound up in a situation I felt like there was literally no way out of. After a while, I just gave up all hope that things would get better because what's the point in wasting the energy trying to get out when you're stuck? I think the same thing could be said for a lot of people who develop depression. I think learned helplessness is a probably an adaptive strategy with an evolutionary basis. I think most people who never became clinically depressed never found themselves in a situation that basically was hopeless if even only temporarily.