Mark Earnest wrote:

I think there are several things going on:

1. It's an attempt to live vicariously through the lives of others.
Most of us aren't blessed with the skill to perform these feats, and
watching (or in my case, fantasy football ;-)) is as close as we can
get to the real thing.

2. We instinctively tend to identify with one "tribe" or the other. The
fact that we live in a particular city is pretty arbitrary after all.
It fulfills our tribal instincts without the danger of real warfare.
Being "for" and "against" things is behaviour that, for evolutionary
reasons, has benefitted us in the game of survival.

3. I think in some cases, it fills some kind of hole that people have
in their lives -- gives them a sense of meaning. For example, in
Nebraska, there's nothing else to do but watch college football. ;-) In
many small towns, they practically shut the town down for friday night
high school football.

Because when they win, you get the feeling of winning.