Getting paid to do something you love can totally ruin the experience.
Odd, eh?
I am vaguely aware that there are many ways of understanding this phenomenon. Many investigators think we have more than one motivational system, and these systems compete - activating one can knock out the other.
One study I've found focuses precisely on this phenomenon. It's called Effort for Payment: A Tale of Two Markets by James Heyman and Dan Ariely, in Psychological Science (Vol15—Num11: 787-793). They were studying "homo economicus", and they wanted to see if adding compensation to a task would affect how much effort people put into a task. If humans are rational self-maximizers, they argued, then the more you pay them, the better they will perform. This is not borne out empirically. In their words: