It's just your basic "us vs. them" mentality. Humans are wired for this (as are other animals), so just about any arbitrary groups you put together will develop this mentality over time. You can even split the groups, mix the members and make two entirely new groups, and eventually you'll have "us vs. them" again.
You see this in sports fandom, in politics, in nationalism, in.... everything.
I pledged a really, really down-to-earth fraternity. Great guys, but no different than any guy in a dorm room. Same people, different living arrangements. That's it.