Consciousness returns to its own dark thoughts and bad memories as reliably as kids to their own scabs, and maybe it's not so difficult to understand why. The mind doesn't like unresolved issues. Except that moods don't get resolved, they get forgotten – but just try forgetting the free-fall through depression's vacuum in a hurry. Worse than that, depression isn't just a memory, it's a state of mind. If you remember it, you're in it.