Part of the reason is that you have evolved. You're no longer 16 years old
Another part though, is that a LOT of sitcom stuff is just repeated ad nauseam. Friends I still laugh at occasionally, but it suffers just asm uch from this as Dharma & Greg or Will & Grace or whatever. Full House, Growing Pains, blahblahblah. They're painfully predictable. Even the good ones! That 70s Show is good, but I can tell you how a given episode is going to end within minutes. And not just those I've seen
It's what seperates the good Simpsons episodes from the bad ones in modern seasons. The bad ones are just "more of the same" or so off the wall in their attempts at humor that they're not funny - too contrived a situation,.... The good ones strike a balance where old characters behave the way they would, but the situation is new or different enough that there's new jokes to be made.
Some newer sitcoms have the same problem. They're so predictable they're as boring as watching reruns. Oh no! Comedic miscommunication giving rise to hilarious hijinks as person A tries to keep person B from discovering it's actually his niece! Ha-HAH! *cough*
Some other newer shows are too temporally bound - political commentary is either so general it's pointless (politicians! Bastards, right?) or won't be understandable/applicable in a few years' time (Obama is the President of Peace, we'll all be so much better off in 4 years time!), new gadgets coming out (an iPod? Wow!),...
Some shows are just a bit weird to rewatch - in Sex and the City, a big deal is made out of not being allowed to smoke indoor in LA, while they can happily do it in NY. "What a curbing of personal liberty! We could never live like that in NY! Haha, silly Californians!". Guess what you can't do in NY these days? Right.
