[saurik]

It's more than just the numbness: the actual numbing agent is often injected in combination with epinephrine (aka adrenaline), which acts as a vasoconstrictor and tends to keep the anesthetic in the right place longer... but some of that is eventually going to get into your overall bloodstream and the result is that you are now a bit anxious/nervous :/ refusing the injection (which I also started doing back about 10-15 years ago, when the mother of my at-the-time-girlfriend told me about her experiences doing that) also thereby leaves you chemically calmer (and likely thereby better able to deal with any pain that does result).