I dont have a source for this, but I've been told a couple times that doctors who get cancer tend to decline chemotherapy/treatment because of what the treatments do to the body. It was described as aging your body to a frail old person (not matter what your initial age was), that is not reversible.

Or think about this on the extreme end: I think many people would choose a time to die peacefully rather than sit in a semi-vegetable state in a hospital bed for the rest of their life.