I had two lightbulb experiences:
1. When I was just starting medical school, I was recovering from a torn PCL in my left knee, and was training for an Ironman distance triathlon. I went to the main knee orthopedist at my medical school, and asked him about help in getting back to Ironman shape. He was very kind, but his consultation was limited to telling me that I was not a surgery candidate.
I went to medical school thinking I would do orthopedics sports medicine, but that made me realize that there was a big gap between the decision whether to surgery, and helping plan the rehabilitation program for the vast majority of athletes who don't require surgery.
I shared my experience with a classmate of mine who had been a physical therapist before medical school, and he recommended I learn more about physiatry.
That's when the lightbulb went on, and I never looked back
2. When I was in my last year of medical school and doing my medicine internship, one of my patients was admitted with some complications of diabetes. Her admission was very short, and when we were discharging her, I was concerned that we were sending her home to an environment that was suboptimal, and that she would likely rebound with further complications. When I expressed this concern to other members of my medicine team (and they were a very good team), it became apparent that they were not really designed to make sure that a patient was discharged to a suitable environment- they were really designed for management of acute crises.
That was my second light bulb moment that confirmed for me that the rehab model was the method of practicing medicine with which I was most comfortable. My experience as a physiatrist has confirmed this- we are really the docs that take care of the little things that prevent problems from occuring or reoccuring, and that is a very satisfying way to practice medicine.