The dentist explained that the upper jaw is more porous, allowing the medicine to penetrate more easily to the tooth. The numbness also wears off faster. By contrast, the lower jaw is denser, so it takes longer for the numbing agent to take effect, as well as longer to wear off.
I think of the numbing agent as Novocain, but I could have sworn the dentist said tequila.
Tequila might have worked faster as well as being a lot more fun than a needle!
No doubt when Sampson slew one thousand men with the jawbone of an ass, he used the lower jaw.
Profound thoughts from the dentist chair. :P