This question is intentionally broad, which allows you plenty of room to make it your own. Your response can be big, like 'helping to grow the minds of the next generation of leaders,' or small, like 'high-fiving my students each morning.' Whichever is true for you will make for a great answer so long as you demonstrate genuine enthusiasm and joy with your response. The interviewers are looking for a candidate with true joy and passion for teaching as well as what motivates you.