I was one of those that made the switch in my Sophomore year of undergrad. First, I'll tell you why dentistry:
As a dentist, you will have incredible opportunities to decide how you want to do business. After just four years of dental school, you can start up your own practice, join with an existing one as an employee, go on to specialize, teach, etc.
I personally enjoy working with my hands. I took several ceramics courses and realized that I wanted to continue developing my skills in artistic esthetics, yet I loved the life sciences. Once I made the connection that dentistry would not only fulfill those desires, but would also be a stable career, I made the switch. Of course, the ability to work four days a week (or more if you want to) and relax on the weekends is an added bonus.
Now, as to why not medicine:
There are several aspects of medicine that are just downright fascinating, don't get me wrong. Yet after I volunteered at a free health clinic, I got a bad taste in my mouth for the redundancy of seeing a patient, diagnosing, and prescribing medicine. It seemed like that's all they did. Granted, it was at a free health clinic so there were no surgeries, etc. I also volunteered at a Cancer Center for a semester and realized that hospitals weren't the place for me. As I see it, hospitals are run by a bunch of red personalities (power-hungry/aggressive) trying to govern a whole bunch of more red personality doctors. It can be ugly. The icing on the cake is that all your patients are always sick. Some doctors I shadowed have said that can be a depressing thing.
I would recommend going into dentistry though with your eyes wide open. Know what you are getting into. Dentistry isn't all roses. Keep in mind that dental schools are more expensive than medical schools nationwide and dentists have high overhead to run an office. There are a couple of decent options to finance your education if you want (HPSP, NHSC, etc). Do some job shadowing or work in a dental environment to see if you would be a good fit for the profession.
To answer your question about how I changed my application to apply to dentistry - it's not that hard actually. Dental schools look primarily at GPA and DAT scores. It is important to have some extra-curricular experiences, but it is not a "deal-breaker." You can still include any healthcare related service activities in your application. However, there is a separate section relating to dentistry/shadowing experience. Look for opportunities to beef up that portion of your application and you should be good.