Willpower is more than just a metaphor; self-control is a finite resource.

If we were designed by engineers, as we consumed more, we'd desire less. But our frequent human tragedy is that the more we consume, the hungrier we get. More and faster and stronger. What was an unexpected pleasure yesterday is what we feel entitled to today, and what won't be enough tomorrow.

In other words, dopamine is not about the happiness of reward. It's about the happiness of pursuit of reward that has a decent chance of occurring.

While this neurobiology is mighty impressive, the brain is not where a behavior 'begins'. It's merely the final common pathway by which all the factors in the chapters to come converge and create behavior.