Since my first kid's first-grade class won some pizza for reading a certain number of books outside class, I've had a hobby-horse about external motivations I ride every once in a while. If you give kids pizza for reading, then when the pizza goes away, so does the motivation to read, and you end up with a bunch of fat kids who still can't read very well.
But I have to remember my own dictum that people vary. Some people really are driven by rewards.
According to this study, people who are driven by rewards (who can be identified by a battery of personality tests as well as by their lateral prefrontal cortex lighting up more often and longer than the rest of us at the prospect of a reward) are just driven in general. In this study, which had them playing word games for zero, 25 cents, or 75 cents, they worked just as hard for no money, and they did better than when it was for money.
That makes it sound as though it's not the reward but the inner drive. Like a gambler, it's not the money, it's the action. Dopamine (or some other hormone) is released somewhere in the brain, and they just feel the flow. Another biological influence on behavior.