Depression, when it's clinical, is not a metaphor. It runs in families and it's known to respond to medication and counselling. However, truly you believe there's a sickness to existence that can never be cured. If you're depressed you will sooner or later surrender and say: I just dont want to feel bad anymore. The shift from despressive realism to tragic realism, from being immobilized by darkness to being sustained by it. Thus strangley seem's to require believing in the possibility of a cure...