When I got asked why dentisty, I related it to my work experience, telling them what aspects of it I enjoyed, i.e. the variation, contact with people, enjoying science aspects etc, - but the important thing is to be sincere about it, remember they will probably be asking that question to most candidates (or a variation of that question), so perhaps relate it to particular parts of your work experience. Oh, - and what ever you do don't mention the money aspect (though admittedly it was probabley my main reason!).
Make sure your honest with what you say in the interview too, - ok so everyone exadgerates a little, but make sure you can follow up anything you've said as the interviewers often pick up on little things and ask follow-up questions (I was caught out a few times). Remember what you wrote in your personal statement, - they will ask about it.
Why not medicine? - I said that I prefered working manually, and that with dentistry you get to build a unique relationship with your patients as you see them on a regular basis rather than only when they're ill. I also mentioned something about dentistry being more specific as your only dealing with the mouth rather than the whole body.