I think an "us and them" is hard wired into our genetic makeup as a defense mechanism. This is a survival strategy that allows us to detect differences, even superficial ones, and put up a fight or flight response. We have not had sufficient time to evolve beyond this. However, as rational beings it behooves us to intellectually counteract this visceral response because we recognize it and intellectually realize its illogical effects. The recognition of it is the result of education and the defeat of ignorance. That is why knowledge and the propagation of knowledge and opinion is so important.