If a child is doing a task they like and we also promise them a reward for doing it, that can only be doubly good for the child, right?
Not necessarily, sometimes the rewards are excessive and can cost us valuable things. Individuals who see rewards as motivation to perform a task may lose the intrinsic interest they previously had in it. This phenomenon is called the overjustification effect, because an activity that is already justified is overjustified by the promise of a reward.
Overjustification can undermine intrinsicness, that is, the desire to do something well just for pleasure, challenge, interest, or self-expression.
Fountain: Myers, D. (2006) Psychology 7th edition. Panamericana Medical Editorial:Madrid