Certainly, I think being depressed is absolutely part of the human condition, it has to be, if there's joy there's its opposite, and it's something you ride if you possibly can.
You've got to be success minded. You've got to feel that things are coming your way when you're out selling; otherwise, you won't be able to sell anything.
It's much easier to write when you're sad. But you can end up isolated and depressed because you almost need to put yourself in that situation to have that angst to write from.