Many people think that in the end, motivation is all about money, for all that people pretend otherwise. If you change the reward, you can change people's behavior.
Others argue that it's all about finding happiness. Duke psychology professor Dan Ariely argues that both play a part, but that those explanations wildly oversimplify things, and miss out on what truly motivates people.
In a recently posted TED talk, he points to finding meaning in work, and being able to see progress as extremely important motivators.