The beauty of being a teacher is that there is always something new to learn. It can be new material about your subject. It can be a new insight on how to present it. It can be a minor revision in your approach to it. Sometimes just when I've finished a class, I realize a way I could have done it so much better. When I'm organized enough, I make a note of it, so that the next time I teach I can utilize that insight.
The primary goal of a teacher is to pass on knowledge. An equally important role, however, is to increase the hunger for learning. Nothing makes my day more than to see students' faces light up with discovery.