I think of it as a contest between me and the weather. Plus, as has already been mentioned, bragging rights, and the astonishment of my co-workers. (Although I suspect it's possible to push admiration over the edge into dismissal, and I'm sure I've already crossed it with some people.)

As someone posted on another active thread (hope it hasn't been posted here yet, I haven't read all these other responses), "There's no such thing as bad weather, just the wrong clothes."