I think that the reasons for choosing dentistry over medicine are the same in many cases. For me personally, I love working with my hands, working in a small group, and having the ability to own my own business. In addition, I despise the hospital setting where people who don't know medicine (just money and capital) tell you what you can and cannot prescribe to patients and insurance is king.
I love science, but both dentists and doctors usually do. Although it is defnitely not the main reason, simply a plus, dentists make good money. I didn't choose dentistry over medicine for that reason, but I can't say that it wasn't a factor. In addition you're not on call, and you set your hours of practice. So I can be on the school board and coach a local little league team if I want to. I will be able to spend time with my family and friends when I want to, not when the hospital says I can.
Overall, dentistry is still much more of a "hometown" type of medicine. you really get to interact with your patients and get to know them well over the years (or at least I will in my small town). I much prefer that. Plus, my experiences with dentistry have been very good; people are usually not in a terrible mood when they see the dentist, sometimes, but not usually. If they are, then we can cheer them up!
Another factor is school. Dentists can practice right out of school, doctors still have an internship and residency (where I've heard conditions aren't so great and neither is the atmosphere).
I respect doctors and what they do greatly, but for me personally, dentistry is the best field to go into.