It is thought that a build up of chemicals like dopamine suppresses the activity of the brain's natural painkillers - the endogenous opioids.

BBC: Gene controls pain threshold

While I go back and forth, I tend to agree that once you get beyond the short term release of endogenous opioids in response to the expectation of pain relief, a mysterious placebo effect that produces all sorts of miraculous effects is far less proven than people think.

FORBES: Harvard Placebo Study Was Seriously Overhyped