The few times I watched Friends, I was pretty much just lost. Nothing made sense without understanding the complicated relationships between all of the characters, and one got the impression that those relationships were so complicated that nobody understood them, especially not the people involved. Like, “This scene is funny because these two people used to be married, until they divorced, and one of them hooked up with this other person”. At least, I think that’s sort of thing that was going on, but in any event, I didn’t get it. It could maybe work in today’s DVR/streaming world of binge watching, but it’s the last thing you want from an episodic sitcom.
Which still put it ahead of Seinfeld, of course, because Seinfeld seemed to be entirely nothing but “This scene is funny because I hate all of these people and want to punch them in the face”, except that that’s not what “funny” actually is.
I won’t necessarily say that Cheers is the greatest sitcom ever (there are a few other serious contenders for that title), but if you sit down and watch a single episode of Cheers, you’ll instantly know everything you need to know about all of the major characters, you’ll instantly see how those characters are interacting, and it’ll be funny. That’s what you’re supposed to get from a sitcom.