I don't know. Sometimes I think they don't even know what they are laughing about. In the newer episodes of the Simpsons they're using the same old jokes over and over again. They also combine it with a stupid and uninteresting story and expect that people are laughing about that. (The Simpson's movie is awful, too.) However, there is one other factor which makes the show worse for me: the dubbing. Marge has now an ridiculous voice and everyone sounds totally fake.
You can't really avoid that shows get worse over time, but as a positive example, I think South Park aged well compared to other popular shows. It is still original and authentic in my opinion, just as it used to be.