When I was in high school I asked myself at one point: "Why do I care if my high school's team wins the football game? I don't know anybody on the team, they have nothing to do with me... why am I here and applaud? It does not make any sense." But the point is, it does make sense: It's a way of building up irrational attitudes of submission to authority and group cohesion behind leadership elements. In fact it's training in irrational jingoism. That's also a feature of competitive sports.
