My spouse almost never gets tired of research. This is a person who loves collaborating with people, helping students, tinkering with equipment, tinkering with data analysis programs, analyzing data, writing papers, reviewing papers, editing papers, planning the next experiment. My son, on the other hand, is cut from different cloth. As my spouse would say, this is a guy who is more of a tool user than a tool maker.
I heard an interview with a surgeon on the radio once. She said, "Don't become a surgeon unless you feel that you can't do anything else." Meaning, unless there's nothing else that would satisfy you. That's the way my spouse is. I don't think anything else would be anywhere near as interesting or satisfying for this person.
My spouse did change sub-fields about halfway through, feeling that the first sub-field was well understood and rather saturated, and wanting more of a challenge. It was a good change.