Caveman A learns how to cut hair.
Tribe then cuts hair short.
Caveman B doesn't want his hair cut, so he keeps his long hair. He becomes noticeably different and alien compared to other members of the tribe. He becomes alienated from the tribe and women of the tribe ignore him because he's viewed as inferior to everyone else. Caveman B dies without producing offspring.
So in short, people want to fit in to the crowd. Wear the same clothes, partake in the same pasttimes, root for the same team, etc. We want to be accepted by our peers because in the past, not being accepted by your peers resulted in you not making babies. Think about it. There was a time when brightly colored short shorts and rollerblading were popular. If you are a girl and see a guy with short shorts and rollerblades nowadays, I doubt you're going to think "Hey, that guy's hip and cool."
But at the same time, there's also a reason why not everyone is the same religion, have the same customs, wear the same clothes, or are organized under one government. While we want to fit in, we also want to be able to stand out a bit. If you and 50 other guys are going after the same girl and all wear the same clothes, root for the same team, drink the same kind of soda, and share a lot of other traits, you are going to have about the same chance of mating with the girl as the 50 other guys. You want to stand out just a bit to make yourself a little more interesting and favorable without sticking out too much to the point that you are fundamentally different from everyone else.
For example, if 65% of a community likes rock music, 30% likes country, and 5% like Polka music; the people who like country will be slightly more unique than than a bulk of the community if all other variables are the same. They won't be too unique since there is still a decent percentage of people who share that interest. However the people who like Polka music are in such minority that if all other variables are the same, they will be viewed as weird and not part of the community.