It’s comforting, no doubt, to think that everyone who disagrees with you does so either out of intellectual deficiency or mendacity; as long as you convince yourself of that, you can believe that there are no really difficult questions and that every problem has an ideal solution. Those particular illusions are dear to most people, and anyone who disrupts them gets punished. Moreover (I theorize) the moderate who disagrees with you is more troublesome than the person at the opposite extreme, since that moderate has probably agreed with you about some issues in the past and has therefore appeared to be a rational person, so the current disagreement seems like a betrayal, and one of the worst kinds of betrayal: the cruel revelation that the world is more complicated than you thought it was....  It disrupts the pigeonholes.
-Alan Jacobs (from here)