Why can getting paid for something you enjoy make you enjoy it less?

Because a reward can change the reason you tell yourself you're doing something. This is the overjustification effect: providing an expected external incentive, such as money or prizes, for an activity that's already intrinsically rewarding can reduce a person's intrinsic motivation to do it, a process often described as motivational "crowding out." In a 1971 experiment by Edward Deci, people who already showed interest in a puzzle were paid for it on the second day but not on the first or third. During free breaks they spent significantly more time on the puzzle than an unpaid group on the paid day, and significantly less once the payment stopped.

One explanation comes from self-perception theory: when a salient reward is present, people may credit their behavior to the reward rather than to their own interest. The overjustification literature often illustrates this with a famous 1959 experiment by Festinger and Carlsmith, better known as a founding study of cognitive dissonance. People did a boring task and then told another student it was enjoyable; those paid only $1 later rated the task as significantly more enjoyable than those paid $20. Paid little, they couldn't easily blame the money, so they inferred they must have found it interesting.

The effect isn't universal, though, and depends on what the reward signals. When rewards reflected competence, higher rewards led to greater intrinsic motivation; only when they didn't reflect competence did higher rewards lead to less. Rewards can even boost motivation for tasks people found dull to begin with, and the negative effect appears stronger in children than in college students. Critics add that lab setups like Deci's, where pay is given once and then arbitrarily withdrawn, don't match real incentive plans, and that there is little evidence on long-term effects.