"Please, let's talk about this anger rather than lashing out - haha, tricked you - POKE!"
It's an obvious reason, of course - a well adjusted person who learns from his mistakes and is intent on improving his life does not lend itself to many laughs. Comedy is all about tension - and if after 10 minutes the characters on The Odd Couple learned to play nice, or The Three Stooges all agreed that they'd respect each other's personal space there wouldn't be much of a show. But what's interesting is more modern sitcoms are on the air for so long, it starts to seem strange if the characters stay exactly the same as they are forever. There are the usual sitcom tropes that often alleviate this somewhat - characters get into serious relationships, they get married, they have a baby (which many also regard as the nail in the coffin for any good sitcom), but those are actually mere scripted sleights of hand - characters can still remain just as needy, neurotic, emotional and catty as they've always been, just under slightly different circumstances. The real problem at hand is that these characters often go an entire series learning the same life lessons over and over and over again only to forget it by the opening credits the following week.
How I Met Your Mother when he started dating love interest (and principal character, which is always key) Robin Scherbatsky, something the writers had been teasing and building up to for some time. Robin finally tamed the testosterone-laden pickup artist, who until that point had treated every other women as a mere object of his desire, a trophy to be won. He finally cared about a member of the opposite sex, a major change for the character. So what happened after a few episodes? They broke up, naturally - and the writers fully admitted it was so they could get old Barney back. Now Barney is back to his old self, without a hint of self-reflection.
Michael Scott of The Office is still just as clueless as he was 7 years ago, but, speaking from nothing more than personal experience, it's definitely starting to feel old. The most recent episode I've seen involved him getting upset over a night at his boss/coworker's (the fact/question that this character was being called his boss being the reason for his attitude) apartment to have a Glee party. His tantrums and desperate attempts to win over his coworkers have happened so many times I can almost immediately spell out how his scenes are going to play out throughout the episode (he'll get upset, try something small, then fail, try something bigger, fail bigger, he'll pout, he'll have a heart to heart, then he'll say something honest and heartwarming but also inadvertently dumb to camera). Inevitably you lose interest - but of course, having said all of this, I don't want to see Michael Scott having woken up one morning, realized how ignorant he's been for this long, and coming to work being a hard working, quiet and respecting boss.
In a sense there's a sort of inevitability built in with great characters like him - you start off loving them from a distance whenever you see them, like your fun drunk uncle, but over time they become the embarrassing alcoholic uncle you wish you didn't have to see so often. In any event, the silver lining is now that Steve Carrell has decided to move on from the role, the character actually has an expiration date, so it should be interesting to see how they conclude his story.
One last example: Jeff Winger routinely learns on Community how important his friends at the college he's reluctantly attending are to him, and how his career as a lawyer was actually not the rockstar life he thought it was. The show even had a moment where he pretended to be "old Jeff" last season after coming back from winter break, a nod to the classic rubber band character trait seen on sitcoms since time immemorial, only to go "just kidding, give me a hug you guys!" seconds later and actually be the changed Jeff he grew to be over the course of the season (the show, if you haven't seen it, is really good about turning these tropes on their head). But this season, to the extent its needed by the plot he actually snaps right back to the un-ironically selfish, vain jerk he was once was (and of course, he'll relearn his lesson to the extent he's unlearned it by episode's end).
There's clearly a balance these shows are trying to strike each week: having a nice, convenient reset button that allows them to start new stories each week and to continue having fun with the characters, allowing their personalities and quirks bounce off of each other, while at the same time acknowledging the experiences they've all been through together and not ignoring major events in their collective lives. Its a fascinating conundrum, and I'm very curious to see if and how they all pull it off.
It's funny - yesterday I spoke about comedic actors having something of an expiration date on playing big, larger than life characters as leads in movies. I think I just realized (I actually didn't mean to connect these two articles at all initially, I swear!) there's a connection here too - with sitcom characters also eventually wearing out their welcome, especially the more emotionally stunted ones. Look at me, inadvertently creating and finding themes!
PS: Final day of 'Post-A-Day: November 2010', where I try something I've never done before!