Dentists are often crooks.
When I was younger, my parents had to change insurance plans, so I got a new dentist. In the first visit, he takes a quick look at my mouth (with no tools, mind you) and confidently says I had 8 cavities that need drilling ASAP.
My previous dentist mentioned no such thing, so my mom took me back to him for a second opinion. He used tools to check, then told us the other dentist was full of shit and that this was surprisingly common.
The new dentist would have drilled 8 healthy teeth out of greed.