A part of it is probably that people who subscribe to defective pictures of the world are less able [.pdf] (via) to recognise when others are off base.

That would explain in no small part why people who hold weird ideas are attracted to other people who hold weird ideas, irrespective of their consistency: Neither is capable of telling that the other is spouting horse manure, and both are opposed by the same kinds of people; that is, pretty much everybody who isn't crazy. The enemy of my enemy is my friend - especially when nobody else wants to be...

- Jake

Friends come and go. Enemies accumulate.