Coz' their dumbasses.
Irony and Stupidity. A+
Dentists are specialists, they know what they're doing. You don't, even if you think you do. They talk to you because they're supposed to be friendly. They aren't you, they don't know how you personally react to a Chemical, and you're supposed to speak up if it doesn't work. Excuse them for trying to make you more comfortable you ungrateful starfish. Let's knock out your teeth with a Balpeen next time, maybe you'll get some sense knocked into your face.