I'd slightly disagree with the 'to help people' thing. You can say that, but if you do, use it as a segue into your answer. IE, I want to help people *with science*...*with the education and hands-on experience medicine provides*, etc. The core of your desire to be a doctor should kind of involve a desire to help people. You just don't want that to come across as your only answer.
But yeah - apart from that, just stay away from "prestige", "money", "power", and things of that nature, and think about why you'd really like to spend four years in med school and several years in residency. Thinking about your final specialty is a good way to come up with an answer (ideally, before you have an interview).