Perna,
That's a great question. I think it's a little bit of both. Maybe there are times when we are depressed but it flies under the radar because things are turning out well-we feel loved, we feel confident. etc... But then it doesn't take much to knock us/me off my feet again. One little set back and I'm back to feeling like I have no purpose in life and I shouldn't exist.
At least that's the way it seems to work for me.