I have nine potential cavities.
As I was sitting in the dentist's chair and having my mouth torn apart, I realized that going to the dentist is exactly the opposite of giving a blow job.
In the first situation, you want to keep your tongue and lips as far from poking objects as possible. I always get the feeling I'm trying to push my teeth away from myself, to get my entire body as far away as possible from the action.
However, with a (nice) blowjob, one tries to get lips and tongue as close to the probing object as possible, making it more enjoyable for both (see How to give a blow job). I personally also try to involve the rest of my body, rather than forcing it away.