Politics and religion make enemies of people who otherwise might get along fine.
Some people seem to be rather insecure in their beliefs and prove it by lashing out at anything antithetical to their little safe place. We're all guilty of it some extent, I'm no exception, it's human nature but some people do so with such venom and spite that it literally sends people walking the other direction.
That's not the only reason but it's certainly a contributing factor.