Eugene Volokh has a characteristically clear-minded post on one of the characteristic differences between left and right (in this case, I think, a difference that carries over from liberalism and conservatism in their traditional forms). It’s a matter of overdoing, or underdoing, the natural human preference for that which is one’s own: self, family, party, country. Liberals, in their fear of chauvinism, can fall into the error of being biased against their own countries, just as conservatives can take patriotism to the excess of chauvinism.
One extension of Eugene’s point: this explains why liberals notoriously form circular firing squads. Criticizing your own side, aberrant for conservatives, is a norm for liberals. It also helps explain the liberal overrepresentation among academics: the academic norm of disagreement is congruent with the liberal norm of self-criticism.