The "overjustification effect" is the cognitive process whereby getting a reward for doing something makes people do it less.
In experiments demonstrating the overjustification effect (such as the classic felt-tip marker study), the rewards were never (that I know of) things the subjects found or discovered themselves, but were presented by the researchers. I emphasize: the rewards were perceived by subjects to come from people.
The process of overjustification is often explained by the idea that a reward acts to shift salience from the inherent enjoyability of the activity to the external-reward-generating aspect of the activity. In short, with the introduction of an incentive, fun becomes work.
Why should this be? Why should the shift to "reward" thinking make the activity less fun?
There is a perfectly rational reason: because the reward is a signal given by interested others, who are one's competitors.
When one receives a promise of an incentive for some behavior, one might rationally ask the promissor, "what's in it for you?" Of course, words can lie, but costly incentives cannot. By providing a valuable incentive (or the reliable promise of one) to someone, the promissor assures the recipient in very credible language that the action of the recipient is valuable to the promissor, and in fact he is willing to pay for it. The recipient should rationally slow down and think about whether, instead of wasting his time doing the activity for free, he should be getting compensated for it - and, relatedly, whether this activity has a cost to him that he hadn't perceived.
One way for this abstract strategy to be implemented would be to cause the actor to feel less joy in an activity another indicated a willingness to pay to promote.
This strategy also applies to disincentives. From a group's perspective, why waste resources prohibiting an activity unless individuals have some reason to engage in the activity? A group prohibition is a reliable signal to individuals that there is some individual benefit - an advantage over the group - to be gained from violating the prohibition. One way to instantiate this strategy would be to build into your creatures curiosity about forbidden things and a little thrill from rule-violation.
The overjustification effect is our rational, built-in stubbornness. We recognize that incentives are signals, and immediately adapt to best take advantage of those signals. Joy is the casualty. The thrill of illicitness is the consolation.