All living organisms are discriminatory by nature, human being is not an exception. By nature they evey living organism has an instinct to help propogate its kind and not someone else's. Why do horses go along with other horses not Zebras because horses have far more similarity in almost everyways with other horses than with any Zebras. It comes down to compatibility. When talking about human. A person of a particular race will have far more common things to share with a peson of the same race than those of a different race. Therefore there is more compatibility between two different people of the same race. However the factors such as social settings in which he/she grew up can make a big difference.