great response Ubtom.
i completely agree with your way of tackling this question. like Jennifer, i too was struggling to answer this question.
as far as nicolas's response is concerned, i think that you didn't understand what jennifer was really asking. we all know (at least hope so) why we want to be a dentist. you would be surprised how many people in my Kaplan course said they wanted it for the $$. now, there is nothing wrong with wanting $$, but thats something you can not say.
this is a hard question to answer. however, if you really sit down and just think about what could you say that will sound sincere and true, then you will be more than OK.
good luck to all.