Many years ago, I came home from work to find my cat apparently unable to fully close her mouth. It was obviously discomfiting her; every few moments she'd make this weird mouth movement that made it look like she was repeatedly adjusting her jaw.

Obviously, I recognized that as a problem, so I took her to the vet, where they did an emergency surgery to remove ... I don't remember the details, but a tooth that had somehow gone awry. They also told me that, though it was not an emergency, a significant number of her teeth were also preparing to malfunction and needed to be removed. They told me that (somehow) this would not negatively impact her quality of life and, in fact, would likely improve it ... So we scheduled a surgery to take care of it.

That day came, we did it, and it is true that I cannot (and could not) notice her seeming less happy. The main thing I noticed was that her breath, which used to be absolutely awful all of the time, now smells fairly neutral.