The main reason is that I think perfection is dull, and makes people dislike you.

There's a video by Ze Frank called "An Invocation for Beginnings," which is one of the few motivational things I've ever found actually, y'know, motivational. One of the memorable lines from it goes something like: "Perfection may look good in his shiny shoes, but he's a bit of an asshole and nobody invites him to their pool parties."