I agree with the above post, but more specifically try to talk about how you got to the point of wanting to become a dentist, the challenges you faced in pursuing your dream, and what you want to do with your degree. This will give them some sort of idea of what kind of person you are and what kind of dentist you might be.
Well I truly believe the essay is for you to tell the school what has led you to want to be a dentist. It should be personal and chronologically describe your path to your application. What events in your life have led you to their door?