I tend to get rather depressed after finishing up a big project. Yesterday, for example, I completed Company for NaNoWriMo, my YA paranormal romance about a ghost and an imaginary friend. The story's fine as far as I can tell. But I still feel sad.
When you're in the process of pure creation, you lear to tune out the inner critic. There's no good or bad, just fun or boring--and if it's boring you force yourself to write until it's fun again. But as soon as you finish, you realize you've only really delayed that judgement. You have a story--now what? Is it any good? Is there a market for it? Is it worth spending the hours fixing it up? Do you dare let anyone read it?
When you're riding high on inspiration--much like when you're dreaming--those bursts of emotion mean so much more than narrative structure or logic. Then you try to explain to others what made it so interesting and they nod vaguely or else give you weird looks.
That's one of the many reasons to reward yourself after finishing something. You may think that the feeling of accomplishment is its own reward, when really the accomplishment is sort of a let down. A tangible reward is better. Mine is to see Catching Fire on the weekend and maybe even spring for some popcorn. If I'm really lucky, I'll go to Barnes and Noble and pick up some new books. Hopefully, that will cheer me up.