The answer is usually related to the why doctor question that almost all the schools will ask you during the interview. I think most of us have personal journeys of why we are trying to reach the goal. Try to incorporate your why doctor answer with things like you'll have more autonomy, and direct responsibility. There's also reasons like leadership, potential international practicing, and that the role of a doctor is more suited to your traits. Don't undermine the nursing profession. Include something along the lines of, "i'm fully aware of the importance of nurses and how vital their role in the care of the patient but ..." You can reword it a little to make it your own.