Why do you want to leave your current position?
The interviewer will be looking for red flags in your answer so no matter what make sure not to speak negatively about your current employer (even if they deserve it!) Be honest but also positive. Emphasize how much you're looking forward to the future and how excited you are to try something new.
"To be honest, I'm just ready for a change. I've been with my current employer for over five years now and although I enjoy so much about the job and the people I work with, there hasn't been much of an opportunity to grow despite my desire to do so. I have gained some experience in mentoring others, and I believe I'm ready to do more of that. Consequently, this position excites me in large part because it'll allow me to manage and lead others. I have done a lot of research on your company and this role and believe it's an ideal fit for both of us."