Sounds like you are saying that the diagnosis in dentistry is easier, and therefore it makes being a dentist less stressful. I would agree, there is less confusion in diagnosis for dentistry, and more surgical (in this case - tooth surgery) being done. There is the occasional cancer diagnosis vs. cinammon reaction that can have major consequences, but much less common than medicine. However, there are certain things that are stressful in any job, and it depends on the situation and the person. You can be a waiter at a restaurant, and all you are really doing is taking orders and delivering food, but anyone who has been a waiter knows it is harder than it sounds. Just like in dentistry, you have different things that can be stressful, like having to redo a crown that has a margin that is open by 0.5 mm. (Keep in mind that you and your patient have spent a few hours and hundreds of dollars on it.) Of course, as you become a better clinician, this will hopefully occur less, but sometimes it is out of your hands (like when the lab screws up), but still affects you.
So, I just wanted to point out that you were focusing on one aspect of dentistry, but there are many more, just like there are many more aspects to medicine, or any job.