Don't analyse it, go with the flow, Shambles. It's to do with solidarity, and that's not the same as nationalism. You don't have to come from a particular locality to support a particular football team - but once you've decided to support one, you're stuck with it for life. It's to do with bonding, like baby ducks...

Anyway, if you haven't got a team to care about in a contest, there's no fun in it. Though having a team you desperately want to lose can be as strong a bond. I once asked mey son if he'd sooner Spurs won or Arsenal lost, and after a bit of hesitation he said it was more important to have Arsenal lose. (Any Americans wading through this, these are two London football teams with a strong tribal antagonism. Well founded antagonism...)