Just reiterating what the commenter above said. The nerve is dead (hence the root canal) so the actual tooth itself won't feel any pain.

In fact, my first root canal was an emergency and they drilled right into my tooth without anesthesia to relieve the pressure. I felt no pain, just relief.

As others have said, it's the noises and sensations... Like you can feel sensations of "something is pressing down on my jaw", but it doesn't hurt.

I think it's also that it feels invasive to have someone in your mouth. It's an oddly intimate part of the body. I had to remind myself that, "I'm consenting to this", and, "this person is skilled and helping me", so I don't get scared or feel like something is happening to me.