The answer is: . endorphin
The endorphin inside our body is a chemical that help us reduce or perception of pain. So, even though the physical damage might remain the same, our mind will make us believe that we can sustain it on higher intensity than what we believe to be.
The blocking of endorphin activity make us unable to reduce the perception of pain. This is why distracting when endorphin activity is blocked would not provide as good of an effect compared to when endorphin activity is at optimal level.
