In "Brave New World," all people are bred and conditioned to fulfill a particular role in society. They are given the amount of intelligence and the conditioning they need to be both productive and happy in their assigned caste.
Bernard Marx is an alpha, a member of the highest caste. For some reason, he is both shorter and thinner than alphas are supposed to be. He is also emotionally different and does not really like a lot of things about his society.
Because he's so different, he feels uncomfortable around the lower castes. Being around them reminds of his physical shortcomings and of the fact that he looks somewhat like people from the lower castes.