The same instinct in a different way manifests itself whenever people with right-wing views brand their opponents "lefties". And, of course, the other way round.
Most humans seem to want to belong to a community. How we define belonging varies, but it seems as if for many people they feel like they belong to one community more if there is some sort of rival community that they can be against. "I am on your side, because they are against both of us."
Sadly, rivalries do matter to many. Even then, for most people they don't take that rivalry to the next level. However much you may disagree with me I know you would never dream of punching me in the face. Or at least, you'd only do it once. For some other people, sadly they do have to be violent. Who knows why? Upbringing, perhaps, or some insecurity so that they feel that they have to prove themselves "worthy".