[mapt]

"Painkillers" describes wildly different chemicals, many of which have no euphoric effect whatsoever. We're not talking about "painkillers", we're talking about opiate/opioid narcotics - derivatives or synthetic reproductions of chemicals created in the opium poppy, which are chosen for their efficacy in crossing the bloody-brain barrier and altering our perception of pain (and everything else), via the mu opioid receptors.

These chemicals stimulate receptors in the brain which are part of natural feedback cycles in which part of our brain synthesizes similar chemicals and feeds them to other parts of our brain to nudge conscious thought in a beneficial direction (learning, eating, running, fighting, mating). The broad class of opioid receptors appears to be central to how the vertebrate brain works.

Natural behavioral reinforcement, pain relief, intense euphoric highs: Same receptors, same effect, same mechanism, somewhat different quantity. Injured people given vicodin often describe "Not caring about the pain". Because they're high - at least, a little bit.