Well, I enjoy reading them. When I go back and do edits and get sucked up in the story even on the fifth time through, I am more confident about it. I also worry that it means I have egocentric tendencies, but I hope that's not the case.
My betas tend to give me good comments even on the things I know are sub-part for me, so it's a good motivator. I used to have alpha readers--people who read the book the first time through as I was writing it, and they would beg me for the next chapter, so that always made it easier to feel that at least there were others out there who would enjoy reading it.
I also know my agent loved at least one of my books, and while it's hard to believe he really thinks it's on the same level as published books, that helps a lot. I do tend to worry that he'll dislike anything else I've written, but I try not to think about that too much.