If you can will yourself to perceive less pain independent of the drug then you don't need the drug. Substitute a sugar pill and convince yourself to feel less pain the same way. That's what the placebo effect is.

If you were developing a new pain medication and in a clinical study you found that 50% of the control group felt less pain and 50% of the group who took your drug felt less pain, you would classify your drug as ineffective, because it doesn't do anything. Not as 50% effective because everyone felt less pain.