My ex wife was a liberal, and I came to the conclusion that she had what I call Perfect Being Syndrome. Liberals, in their minds, are supposed to be good people and perfect in every way. Humor points out the quirks and imperfections we all have. Conservatives understand that we are all imperfect and we can laugh at ourselves and others. Liberals, on the other hand, see it as a personal attack. You are questioning their perfection.
I think you are on-point of the root of all this. In itself, the self-expectation of being perfect, is depressing and impossible.