Does it seem to you that when you get paid to do something that you love to do, the activity then becomes less enjoyable?
Here's an excerpt from "Swearing is good for you: the amazing science of bad language," by Emma Byrne:
Give most children crayons and paper and they'll happily draw for the fun of it; the intrinsic reward of doing something creative keeps them happy and interested. But as soon as you pay children for their art, their drawings get sloppy and less detailed. They also don't seem to enjoy the process of drawing anywhere near as much as when they are offered a treat in return for each piece produced. In studies, children who know they will be rewarded for their drawings spend only about half as much time playing with crayons as those children who aren't offered a reward.
-page 124
The author references M. R. Lepper, D. Greene, and R. E. Nisbett, "Undermining Children's Intrinsic Interest with Extrinsic Reward: A Test of the "Overjustification" Hypothesis. Journal of Personality and Social Psychology 28 (1973), 129-137. doi:10.1037/hoo35519