The great thing about episodic television is that it allows writers to really flesh out every character, which, in turn, allows viewers to get more invested in their stories. For the most part this is a huge positive. And, in fact, it's become the preferred method of storytelling for many of Hollywood's writers.
The unfortunate side effect of evolving characters over dozens or even hundreds of episodes is that, whether the showrunners want them to or not, these characters sometimes transform in ways that make the general public want to reach through their TV sets and strangle them until their knuckles start to cramp.
Because television series are like relationships: In the beginning, your partner is pretty much idyllic, to the point you're convinced the only thing that comes out of their butt is rainbows. As you spend more time with them, however, you notice they leave their toenail clippings on the kitchen counter and refuse to watch all Mel Brooks movies.
It can really sting.
So it is with your once-beloved television characters. Eventually, those rainbows fade away and you have to hold your nose every time they appear on screen.
Maybe the writers planned it, or maybe they just couldn't figure out how to sustain a lovable character for 10 seasons. Whatever the case, some truly wonderful, fictional people have been utterly ruined by the time of the series finale.