Now I learn that I am apparently out of step from an evolutionary point of view. (No comments necessary.)
According to an article in the Journal of Evolutionary Psychology, following sports teams is the "by-product of an evolved coalitional psychology."
In other words, rooting for the Red Sox and hating the Yankees is what's left of the set of skills required to get the Cro-Magnon in the next cave to help you steal a mate from the guys on the other side of the river.
In fact, the psychology is so deeply set that sports fans behave as if they were actual members of the team they're following. So it isn't enough for them to love the Red Sox and hate the Yankees, they have to hate anyone whose affections go the other way.
Sadly, these days, that evolved coalitional psychology applies to politics as much as to sports. So maybe I'm not missing too much after all.