First, as an undergrad I hated kinetics. It was all boring equations and seemed unrelated to reaction mechanisms. (Bad teaching? Bad student?) Anyway, I discovered that once you care about the reaction, kinetics was fascinating, and I was good at doing consistent experiments and detective work.

Second, I had taken no biochemistry or even biology since 10th grade. It never occurred to me chemistry was related to biology, and once it did, I still wasn't very interested. That all changed in grad school. Not sure what the lesson is besides keeping an open mind and trying new things.