handful of seasons is that in the last few seasons, characters start becoming caricatures of themselves. Writers run out of ideas and make the character quirks the focus of the show. It definitely happened with Friends, and to a lesser extent, The Office. Joey got dumber, Monica became even more OCD and annoying. Ross is more socially awkward. In the Office, Andy and Angela certainly go through this. Even Dwight is not immune. Sometimes it leads to funny moments. Usually it's just bad writing. I haven't watched a full episode of Seinfeld for a long time. Perhaps, it's an issue too. From memory, though, it didn't get quite as bad in Seinfeld.