For a variety of reasons. the pulpit rabbinate is a high-stress job.
First of all, and most obvious, I would think, you have to deal with more illness, cosmic unfairness, death and dying than almost anyone else except physicians. Being on call 24/7, and having to be strong and composed for others who are suffering and/or grieving exacts a tremendous toll in both the short and long run. Taking care of one’s own inner life as a rabbi is an under-appreciated challenge. The accumulated grief wears you down.