Reward prediction error governs effort and learning by altering neural circuits when dopamine is released.

“But reward prediction error also says that if you tell the kid, or yourself, "Okay, we're headed to the amusement park." "We're going to get some ice cream." They're really, really excited. And then you get there and it's closed, or they're not letting any more people in, well then the dopamine level drops way below what it was before you told them that you were headed there. Okay? I've given a number of different examples that hopefully make this clear. Reward prediction error governs virtually all aspects of effort and all aspects of learning.”