I think it has to do with your brain wanting to group people together.
I also think that society is being more and more homogenized. People are so goddamn similar everywhere that it makes me sick. You talk to a few 20 something white females and you've talked to most 20 something white females. You talk to a few 50 something white males of a certain income and you've talked to them all. Originality is rarer and rarer.
Everyone under 30 today spends all of their time talking about B.S. and typing away on their blackberries or whatever they are, and texting each other. They all dress the same, eat the same, talk the same, think the same. It's sickening.
Fuck you world!