Dozens of psychological studies have consistently shown that giving expected extrinsic rewards for an activity (e.g. "If you do x, I will give you y amount of cash/points/...") often reduces intrinsic motivation of people to do it. The first reason is that people feel controlled by the person giving the rewards, reducing their sense of autonomy... Secondly, giving a reward for an activity sends a strong social signal that you don't consider the activity worth doing for its own sake.
-- Sebastian Deterding, "Don't Play Games With Me! Promises and Pitfalls of Gameful Design"