Humans like animals have a biological drive that motivates us to eat, drink and copulate. It's the most basic form of motivation that keep us alive and well. We are also motivated by extrinsic elements like rewards and punishments. These extrinsic motivations are the basis of our business world. You want something done better and faster, you pay more. If you want to prevent a certain behavior, you punish them.
The third drive is by far the most powerful form of motivation. It's intrinsic motivation that drives programmers to release open-source software and made Wikipedia the best encyclopedia in the world. They are doing it for the enjoyment of the task itself. The performance of the task is in itself the reward.
This is the reason why you should be doing things that you love. Intrinsic motivation is both renewable and limitless.