Slashdot not withstanding.
Have you noticed how both sides of the political spectrum exhibit this trait so eloquently? They both talk as if they have a mandate from God (pun intended) or that they invented the Truth (again, pun intended). Look closely and you may not like what you see.
I think that it has a lot to do with human nature. Also seems like it would fit well as an evolutionary trait to ward off cognitive dissonance.
It has been posted before but bears repeating. Treating the other side with respect and realizing that they may have a point will go a long way to tone that down.