Why Jokes Are Funny to Some People and Not Others

"A priest and a rabbi walk into a bar... " You have heard this joke set up a thousand times. Before the joke reaches its punch line you're smiling and the guy next to you is moaning. "Knock, knock?" Unless the joke teller is a third grader with a lisp you know this riddle will end with a thump. Of course if you are the proud grandma this will be the funniest joke you ever heard and slip the future comedian a buck.
So why do people respond differently to jokes? How can a joke that causes one person to snort and spit and practically whiz on the floor, not even make another person crack a smile? Uhm, sorry about the crack reference.