I'm not one of those "I always knew I wanted to be a pediatrician" people. No one specific experience either. I enjoyed a few other rotations during third year, but never really connected with adult patients. My IM rotation was awful, really. I also had trouble finding much sympathy for a lot of adult patients. When I thought about the kinds of patients I wanted to interact with for the rest of my career, it was clear that peds was the way to go. There was also the influence and mentorship of the pediatricians I met during medical school. My personality just seemed to 'fit' better with that group, along with the way I thought about problems. Unlike the IM guys, they didn't take themselves too seriously but were just as smart. They were people I wanted to learn from and interact with.
Just my experience.