What I am trying to show you is this: that we laugh only when there is some reason which is forcing us to laugh. A joke is told, and you laugh – because a joke creates a certain excitement in you. The whole mechanism of a joke is: the story goes in one direction, and suddenly it takes a turn; the turn is so sudden, so drastic, that you could not have imagined it. Excitement grows and you are waiting for the punchline. And then suddenly, whatsoever you were expecting is never there – something absolutely different, something very absurd and ridiculous, never fulfilling your expectation.
A joke is never logical. If a joke is logical it will lose all its sense of laughter, the quality of laughter, because then you will be able to predict. Then by the time the joke is being said, you will have reached the punchline because it will be a syllogism, it will be simple arithmetic. But then it will not have any laughter. A joke takes a sudden turn, so sudden that it was almost impossible for you to imagine it, to infer it. It takes a jump, a leap, a quantum leap — and that’s why it releases so much laughter. It is a subtle psychological way to tickle you.
– Osho: The Dhammapada: The Way of the Buddha Vol-8

A beautiful girl was talking to her psychiatrist about her problem. “It is liquor, Doctor. Whenever I have a few drinks I have a compulsion to make love to whomever I happen to be with.” “I see,” said the doctor. “Well, suppose I just mix up a couple of cocktails, then you and I sit down, nice and relaxed, and discuss this compulsive neurosis of yours.”