Justine Musk (Elon Musk’s wife) had this as part of her response to a question on Quora about how to be as successful as Bill Gates, Steve Jobs, Elon Musk, and Richard Branson:

Be obsessed.

Be obsessed.

Be obsessed.

If you’re not obsessed, then stop what you’re doing and find whatever does obsess you. […] Don’t pursue something because you “want to be great”. Pursue something because it fascinates you, because the pursuit itself engages and compels you. Extreme people combine brilliance and talent with an insane work ethic, so if the work itself doesn’t drive you, you will burn out or fall by the wayside or your extreme competitors will crush you and make you cry.

Intrinsic rewards over extrinsic rewards.