Why can an inactive placebo still change how someone feels?

Answer: Expectation changes brain and body signals

It secretly contains a little medicine — A placebo is defined by not having an active treatment ingredient. In trials, that blank treatment is useful precisely because any response has to come from the treatment context, expectation, conditioning, or normal symptom changes rather than hidden medicine.

Expectation changes brain and body signals ✓ — Expectation is not imaginary. The ritual of treatment can change pain, reward, and stress pathways in the brain, including the release of the body's own opioids and dopamine. That can make symptoms such as pain or nausea feel genuinely better, even though it does not directly attack the cause of a disease.

Willpower removes the underlying disease — Willpower can influence attention, stress, and how symptoms are experienced, but it is not a cure by itself. Placebos work best on symptoms the brain helps shape, such as pain or anxiety; they do not shrink tumors, kill bacteria, or replace real treatment when the underlying disease needs one.