Surprise is a mental "slap in the face" that leads people to laugh.
Most assessments of humor's underlying structure gravitate to the notion of controlled incongruity: You're expecting x, and you get y. For the joke to work, it has to be readable on both levels.
People only laugh when there are others around. It's a form of communication. That's why it's so tough to kill in a room with only a few people.
"You're 30 times more likely to laugh when you're with other people than you are when you're alone...In fact, when you're alone, you're more likely to talk out loud to yourself than you are to laugh out loud. Much more...We've vastly overrated our conscious control of laughter."
You can't tickle yourself. You need an "unpredictable touch." Punch lines need to deliver that same sort of unpredictability.
Like the incongruity theory of humor, tickling relies on a certain element of surprise, which is why it's impossible to tickle yourself. Predictable touch doesn't elicit the laughter and squirming of tickling — it's unpredictable touch that does the trick...The laughter of tickle evolved as a way of cementing the bond between parents and children, laying the foundation for a behavior that then carried over into the social lives of adults. While we once laughed at the surprise touch of a parent or sibling, we now laugh at the surprise twist of a punch line.