Jeez. Is there any reason they keep changing doctors? Do they keep quitting?
There have been a grand total of 33 seasons spread over the last 50 years. Some change of faces is not unexpected (much like James Bond). Really though I do not know why the first set or two of doctor changes happened, that was way back in the 60s and I couldn't make myself watch that if I wanted to. Now it is kind of expected that each doctor will only run for about a half-dozen years or so at most. It is not just an actor change though, it is built into the story line, and while the 'character' is the same, the change also greatly impacts his personality and mannerisms, the writing staff uses it to take the show in new directions as well. So when we talk about liking X dr, it is not really about just liking that actor, it is about liking the character directly, and liking that era of writing indirectly.