Arch-conservative blog Ace of Spades (which is frequently as mean as the stuff you might read on Atrios...) has written a persuasive (and loooong) post that seeks to understand why Leftists 1. Hate Israel and Conservative Blacks, and 2. How Leftism often leads to its own brand of insidious moralizing.

Leftism, and liberalism, and progressivism, and etc-ism. are not merely simple politics for most of these people. Their politics to them are a core part of their identity, and, more importantly, a central support propping up their egos. They are enlightened because they believe these things; someone who does not believe these things, and yet who, superficially at least, appears to be about as smart as they might be, represents a threat to their egos. The foundation upon which a crucial structure of their sense of self-worth is undermined if they discover that there may be people who can pass as normal and intelligent and yet do not believe as they do.

If one is smart, then one believes in progressivism.

If one believes in progressivism, then one is smart.

Those are the two assumptions that prop up their sense of self worth, and they are refuted by examples of smart people who don't believe in progressivism.

And because there is a great deal of personal psychological investment in progressivism, they react intemperately to rejections of it. It's not merely a tax cut that's being debated; it's they're very sense of importance that's being attacked. It's not merely gay marriage which is being argued against; it's their value as human beings that is being uncouthly denigrated.