You hear a joke, and you laugh yourself blue. Boy it was funny. You repeat it for some friends, and you can hardly tell the punch line with a straight face. You hear it a few times the next day, it's making its rounds of the office. You grin; you start to mention that you've heard it. It starts to get boring. Why should that be so?
You keep on eating mom's apple pie; it doesn't begin to taste bad after you've eaten it a few times. Sunsets and rainbows don't get boring because you've seen one before. But humor is that positive self-reinforcement you get for learning a lesson, and it wouldn't be an effective strategy to keep rewarding yourself for learning the same lesson. You need to go on, and learn something new. So the joke begins to get boring. Even types of jokes get boring, making faces is not as hilarious as when you were a child, you lose your taste for making fun of people's names, and references to excrement are churlish, not witty. Slapstick can become ho-hum, perhaps even puns begin to pall.
Of course, some people seem to get stuck on particular forms of humor, there are those who laugh at the mention of shit throughout their life, or who can't see someone getting hit by a pie without cracking up.
I'm not sure whether these people are perceptive enough to recognize that they havn't completely learned a necessary lesson; or whether they are desparately trying to hold onto a pleasure that they once enjoyed.