I think the nature of human laughter deals with surprise as the main element.
ALL comedy, written or verbal, has to do with deception (setting up the joke) and surprise (the punchline). GOOD comedy requires timing and assessing your audience (they have to get the joke).
"A rabbi, a priest and a journalist walk into a bar...." The punchline is always going to be what you didn't expect. The more surprising or unexpected the funnier (it doesn't even have to make sense, it just needs to catch you off guard). Hate the kind of humor you might find in a joke book, but it's just to illustrate a point. Taking seemingly unrelated things and connecting them in a new way is also surprising. This framework is necessary for anything that is funny. Narrative humor, cartoons, or any kind of humor, works using the same idea.
Woody Allen isn't funny because of self mockery but its' the way he does it. One of his lines was "with a body of mine you don't get jealous." The self mockery isn't funny in itself, it's the implication (the surprise) that he has a great body. If he said, "with a body like mine, I rarely get dates." That isn't funny - even if it's self mocking.
This also applies to slapstick. A guy is walking down the street minding his own business and then "BLAM!" a safe drops on him, or he gets a pie in the face.... Surprise you weren't expecting that! Whether you find this funny or not is another question, but plenty of other people do.
When I was high school I used to have this habit of calling random people squirrel, squirrel-boy, ferret face, rat-boy, weasel hair, whatever. It made no sense whatsoever, but made people laugh anyways because it was completely unexpected.
