I have always wondered what passion exactly is.
Caveat: Dental Student, not Dentist. But I'll explain why I went for D-School despite not feeling "Passionate" about dentistry: It's because I don't really get "Passionate" about things the way books and movies generally talk about "Passion" - that is, an unwavering love and willingness to devote every waking hour. Dental School is a path which will open a whole lot of new paths (see the Navy post above mine), many of which seem intellectually stimulating to me. The Dentists I shadowed assured me "If you're bored, you can get out and do something else with your DDS. You don't have to just do one thing. People who say Dentistry is boring are boring people." That sounds like an ideal job for me - something constantly in flux so I can keep my restless brain engaged for a lifetime.
The problem with the question "What is Passion?" is that it's nebulous. It's cousin to the question "What is Happiness?" Happiness and Passion are both custom-tailored to your life. It's too personal to ask the question "Can you evaluate my level of passion? Am I passionate enough about dentistry to make it through the next forty years??" How could anybody know?
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The question about Passion becomes increasingly problematic when you add the fact that there are going to be days, no matter what you do, when you wake up and go WHY AM I DOING THIS OH MY GOD KILL ME. Think about playing video games (or insert activity you enjoy here) from 9 to 5 every day. Heck, think about having sex every single day (jokes aside, most couples don't do this). Does that mean you're "not passionate enough" about video games and sex? I'm doubtful
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I think "Dedication" or "Perseverance" are more specific and useful terms in general than "Passion". If you find yourself intellectually stimulated by the study of medicine and surgery, if you find yourself desiring the ability to have a tangible impact on your community and the people around you, if you find that the idea of working with mouths doesn't turn you off, and if you're willing to work hard through your life to provide a consistently high level of service, I think you're going to be just fine, because Dentistry rewards hard work with multiple options in an intellectually stimulating profession.
Of course, if that's not true, I'm boned, and I guess I'll hate the next 40 years of my life.
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Ending Caveat: If you HONESTLY find yourself telling yourself "Dentistry looks like it sucks but I want a 30-hour job that brings home the loot", get out now. You don't have to tell anyone, but do yourself a favor.
EDIT: Answering second half of the post, about learning more about dentistry
Definitely go shadow. You don't necessarily have to work as an assistant but if you can, it helps. Shadowing and talking to dentists really let me know that Dentistry was something I could commit to. Think of every question you would want to ask here and go ask a couple of Real Live Dentists too. That way you can get a ton of different perspectives.
Try and shadow a dentist in a clinic, a dentist in a hospital setting, a dentist in a rich suburban private practice, a dentist in a 'mill' private practice, etc. Get perspectives from as many types of Dentists as possible - they're not all the same. If you only talk to the owners of Boutique Veneer practices just to log 1000 hours of (pointless) shadowing, you're doing yourself a disservice.