I bike commute in winter because it's faster and warmer than the bus. Not quite as warm and almost as fast as driving (not that I have a car anyway), and not counting parking and scraping ice off the windshield and all that jazz.
I bike commute in winter because I'm usually in a better mood when I get to work. Sometimes I get to see things like a bunch of kids playing with a remote controlled toy snowmobile. I had no idea such a thing even existed! (Then I got distracted, misjudged some ice, and bit it, but luckily nobody saw me, as they were all looking at the toy!)
I bike commute in winter because I've got to do *something* halfway impressive and/or bad-ass to salve my ego and keep myself going when the demons pipe up about failure. Hey, demons, I rode my bike in a freaking snowstorm today, go away, I'm tougher than you! Seems to work so far.