Humor has to do with personal frame of reference. The humorist establishes a context with his audience, leads his audience along the lines of said context, then surprises his audience by taking the context in a different direction. This isn’t the only type of humor, of course, but it’s the most common. Conservatives and liberals in the modern era very rarely laugh with one another because we are so far removed culturally from one another that we no longer share the same context.
Because of minimized personal contact and an unprecedented ability on the part of individuals to filter those with whom they interact we barely even share a common culture any more. When a liberal makes a joke about “Chimpy McHitler’s Illegal War For Oil” other liberals will laugh because they live in that sound chamber: it’s what they read on their blogs, hear from the commentators on the liberal news sites, talk about with their liberal-only friends. Likewise, when a conservative makes a joke about “HopeChange Obama and the Hope for Change” other conservatives laugh because their blogs, talk show hosts and conservative friends have been talking about the same thing. We’ve contextually established ourselves as liberals or conservatives, and therefore can no longer laugh at the same jokes in a political context.
It would be easy to mistake this for only laughing at what is true, because, of course, you believe the truth. In reality, we laugh at what is contextually relevant to ourselves.
