Different people react in different ways to all kinds of things. It’s possible they are doing it right (whatever that might mean), but it has limited effect on them.

Case in point, there’s at least one numbing agent that wasn’t quite enough for me at the dentist when I needed some deep-ish work doing. Two doses of the regular stuff didn’t do the trick for the tooth in question - despite the fact I was drooling all over the place and probably looked like I was having a stroke - so they had to dig out a scary looking syringe and some nasty tasting stuff.

I also had gas once as a kid and I don’t remember any high from it, but then does anyone get a high from the knock-out dosage a dentist gives?