You don't think you're hungry, then a friend mentions how hungry he is or you smell some freshly baked pizza and whoaaa, you suddenly feel really hungry. Or, you've had surgery and need a bit of morphine for pain. As soon as you hit that button you feel relief even though the medicine hasn't even hit your bloodstream.
These are two examples of the oft-studied placebo effect that demonstrate the amazing and still somewhat confounding powers of the human brain.