I'm not talking about dental specialties (and neither were you in your original post), I'm talking about dentists and dentistry. I'm also not denying the fact that dentists are highly trained, highly skilled people whose work is valuable and important.
What I am saying is that by becoming a dentist you are learning a skill at university that you can exchange for cash. Given the renumeration and job security offered in dentisty, I think the fact that all you need to do is get into the university degree and pass, makes the whole thing relatively easy.
To get similar renumeration and job security in business or commerce, in my personal opinion, seems more difficult. To become fully qualifed as a medical specialist, also, is a more difficult and complicated endeavour.
My point about people getting the dentistry service elsewhere is this. By becoming a dentist you are exchanging skills for cash. Since you are being renumerated for your work, it's akin to a business transaction. If one particular dentist didn't provide that service, the person could get the same service from another dentist. That is to say, your role as a dentist isn't a world-changing role, if it wasn't you doing this 'help' it'd be someone else (another dentist). Thus I don't think dentistry is about helping people, I think it's about exchanging goods for services.
Medicine, in many cases, is likewise. Aside from surgery, however, I think there is more opportunity in medicine to go above and beyond your call of duty and help people. Not much more though, hence why I don't think 'wanting to help people' is a particularly good reason for wanting to do medicine.... unless you really intend to go above and beyond your duty.