The Buddha said, “There is no way to happiness. Happiness is the way.”
Spinoza said, “Happiness is virtue.”
I see a pattern emerging.
I think time flies when you’re having fun because that’s what you’re supposed to be doing, so the universe gives you a little hand. The faster time seems to go, the fewer decisions you have to make in the same amount of time. Therefore, the chances of your screwing up your happiness with a poor decision are lower.
For depressed people, time seems to move slower. I think this is the universe giving us a hand too; the slower time seems to move, the more opportunities we have to make decisions that help us climb out of our unhappy predicaments.
Make sense? I hope so. 
–Mike
