Andrew Sullivan writes about leftists and anti-semitism:

The answer, I think, lies in the nature of part of today's left. It is fueled above all by resentment - resentment of the West's success, resentment of the freedom to trade, resentment of any person or country, like Israel or Britain or the U.S., that has enriched itself by means of freedom and hard work....In the West, parts of the left, having capitulated to moral relativism and bouts of Western self-hatred, have seized on Israel as another emblem of what they hate.

Eugene Volokh argues that a lot of criticism of America and its allies stems from the belief that it's clever:

We are all taught, or at least should be taught, to resist uncritical acceptance of what one's country, culture, or religion does. That's a virtue. But many people take this to the point that all criticism of America and its allies is seen as noble and thoughtful, and all defense of America and its allies is seen as jingoistic or naive. This is tempting, but it's just as much a vice as excessive patriotism; in fact, it is blind patriotism's precise equivalent.

(Links via Instapundit, who agrees.)

First of all, I think there is a tendency for some to label any criticism of Israel as anti-semitic (a kind of over-reaction that one sees on the left as well). But as far as leftists are concerned, while their outlook is partly due to moral relativism, another element is not so much Western self-hatred as guilt about having it so good, based on the faulty assumption that our economic success is a zero-sum game. Many leftists mistakenly believe that Britain and the U.S. have enriched themselves by exploitation rather than by means of freedom and hard work.