Actually, wait a second.

Why don't we become immune to our own endorphins?

What mechanism is setting that acclimation, and why can't we adjust that instead of endorphin levels?

Like, you know how when we use heroin or morphine or something, natural endorphin production adapts and goes down to try to reach a certain level?

Why can't we target the thing causing the adaptation and just set it higher? Then we wouldn't even need to take drugs, the body would just pain kill naturally.