This occurs where I attribute my behavior more to a conspicuous extrinsic motivator than to intrinsic reasons. This effect is less when rewards are given for performance success rather than simply completing tasks, but can still be significant.
Greene, Sternberg and Lepper (1976) played mathematical games with schoolchildren, which the children seemed to enjoy. After a while, they started giving rewards for success. When they took away the rewards, the children quickly gave up playing the games. [Also those children who were not rewarded gave up the game less quickly].
The explanation was that the children had decided that they were playing for the reward, not for the fun.
I play a roleplaying game which does not lead to particularly good play sessions. I do it only because I have been trapped into collecting experience points.
Perhaps this is why I don't play much d20 if I can help it.