If you don't believe me go to your local high school. If your school has enough kids of different races you will see that they all sit together by race, you might see one or two exceptions, but for the most part this is the rule.
That's because people are more comfortable around other people who are like themselves.
I went to a very mixed public high school (mostly whites and blacks), and though there was no forced segregation policy, the kids voluntarily kept very much apart. The exception? Those white kids who had accepted the black culture. Again, they were more comfortable around people like themselves -- who happened to be black.
But it's not just about race. I wouldn't be comfortable or close to a man who makes $300,000 a year either -- unless his lifestyle was unusually down-to-earth.
Scripture even deals with this:
Ecclesiasticus Chapter 13
2 He shall take a burden upon him that hath fellowship with one more honourable than himself. And have no fellowship with one that is richer than thyself. 3 What agreement shall the earthen pot have with the kettle? for if they knock one against the other, it shall be broken. 4 The rich man hath done wrong, and yet he will fume: but the poor is wronged and must hold his peace. 5 If thou give, he will make use of thee: and if thou have nothing, he will forsake thee.
...
19 Every beast loveth its like: so also every man him that is nearest to himself. 20 All flesh shall consort with the like to itself, and every man shall associate himself to his like.21 If the wolf shall at any time have fellowship with the lamb, so the sinner with the just. 22 What fellowship hath a holy man with a dog, or what part hath the rich with the poor? 23 The wild ass is the lion's prey in the desert: so also the poor are devoured by the rich. 24 And as humility is an abomination to the proud: so also the rich man abhorreth the poor.
Notice this goes for religion as well -- a Catholic will feel most comfortable around other Catholics -- presuming the Catholic Faith dictates his everyday life (as it should).
Matthew