There used to be an old saying: "Everyone's strange but me & thee. And sometimes thee acts funny." I would presume this may well go back to the earliest development of our species. There would have been many eons during which having anyone not be in "lock-step", so to speak, with the group would have hindered if not endangered the whole clan. So chances are there are still portions of our brain that are wired to resist any kind of behavior that's outside the established norm. At least that's my personal, non-professional perspective on the subject.