Laughter is thwarted aggression

One of those Freudian things; we hate knowing.  We laugh when we discover an internal aggressive impulse toward ourself or friends. Jokes are based on the principle.  The joke leads into a double meaning, one socially bad and one socially good.  The listener develops disgust at the social improprietary hinted at by one meaning.  Then the punch line reveals the more mild meaning and the aggression is released by laughter. Often the joke will imply an unsocial act, then the punch line places the act in a context the listener recognizes as himself.  His aggression at himself is thwarted into laughter. The joke, like the dream, disguises the social improprietary.