The answer to "why dentistry?" is really a personal preference thing. When I went to my interviews...the main thing I've been told by the adcoms is to be honest and don't feed them lines like I want to be a dentist because I want to help people. That's apparently one of the most cliche lines and they don't want that.
Think of why dentistry is right for you and why you're a right fit for the field (and to study at their school for it). My generalized reasons: lifestyle, type of patient care, autonomy, being able to work with my hands. It doesn't have to be grand idealistic reasons....it just has to be YOUR reasons for choosing dentistry.