I just read an interesting article on Slate about why people love to root for the underdog. I support the White Sox and the Brewers against any other team (when they play each other every three years, I root for the White Sox), but if I have no real knowledge of the teams involved, I'll root for whoever is losing. I know some of my friends do this, too.
That didn't strike me as weird until Daniel Engber, the author of the article, noted that it's counter-intuitive that we would always root for the losers, because it means we're more likely to be disappointed. He goes on to examine this and summarize several interesting studies. A lot of it relates to the ideas brought up in A Beautiful Math, that book I read on game theory.
It's an interesting article. Here's the link: http://www.slate.com/id/2252372/