Sadism, would be my guess.

Long before Novocain, the ancient craft of drilling - The Boston Globe:

But Dr. Richard Glenner, a Chicago dentist and author of dental history books, said the drilling could have been decorative or to release ''evil spirits“ rather than fighting tooth decay, he said, adding: ''Why did they do it? No one will ever know.”

I mean, did you ever see the Marathon Man? That was nothing, I tell you. Nothing. I used to go to a dentist in the Village that had some kind of antique drill set I think he maybe used to pump with his foot, I don't know. But I remember the smell of the smoke it made. He wore one of those green eye shades and didn't believe in the foofoo stuff, like Novocain. But he was cheap.

Anyway, it turns out, this kind of thing has been going on for 7,500 years or more. In Pakistan. So maybe we ought to just let them keep Osama and work on his cavities. The last guys they worked on have carbon-dated skulls. Ouch.

''Definitely it had to be painful for the patient,“ Macchiarelli said.