It just seems like a rite-of-passage for any TV show that lasts more than about four seasons to Jump The Shark. Usually, the characters just change before half a decade is over . It is essentially the right amount of time for things to get different just like in real life.
People get older, they move , they think differently about their friends etc.
Often for TV characters they change too much and are not the individuals who everyone liked or loved at the beginning