If you've ever asked yourself why you're a sports fan, and have been unable to come up with any kind of a satisfactory answer, the Associaion for Psychological Science is here to help. And I'll admit it... I have no good explanation for why I let Ryan Leaf ruin so many of my Sundays. It just seemed like a good idea at the time to get emotionally invested in Ryan Leaf.
The APS people narrow it down to a few reasons. You have a need to belong, to be social, and to feel like a part of a group. You identify with your team. You feel like your loyalty will be rewarded in some way. And, perhaps most interestingly, because you see your favorite team as an extension of yourself, your own personal self-esteem gets a boost when the team wins.
An early-90s study took a group of college basketball fans, and sat them down and showed them tapes of their teams winning or losing. If their team won, the participants not only rated the team's prospects for the future higher, but they perceived their own prospects for the future as better. Their self-esteem got a boost when their team got a win, a boost very similar to what a person would receive if they succeeded at something themselves.
No wonder Yankees fans can be such arrogant bastards.
Sports Complex: The Science Behind Fanatic Behavior [aps.org]
Shrinks ponder the psychology behind the phrase 'Yankees Suck' [Rammer Jammer Yellow Hammer]