I was an EFL teacher for 5 years prior to teaching CELTA, and have taught CELTA for about 10 years.
I like surprising students, at least keeping them on their toes, keeping challenge levels high and the focus on dealing with the challenges on the students. This requires flexibility and constant experimentation in lesson planning – each lesson is a kind of problem that requires its own solution. That, for me, is what is most rewarding about teaching