It's called: YOUR BRAIN.
You do something. it is something "good". Your brain rewards you with a dopamine. This encourages you to do more. You do more, youget another dopamine hit. Eventually you reach the best you can be. The dopamine wears off.
You either:
1. Move on.
2. Learn to push forward, finding some reason to do X that isn't just your brain's architecture that is designed to encourage you to learn useful skills.
3. You try harder and harder until you give up, then you move onto something similar. You start doing whatever it takes to get your next high. Depending on the situation this can turn quite ugly. This is how people become Bullies, Corrupt politicians, and Addicts to Various forms of Vice.
Brain function wise, there really isn't any difference between an Addict, a Corrupt Government Offical, or a Bully.