Hard work followed by a reward is a powerful motivator, whereas rewards given without effort or achievement are detrimental. That's why participation trophy for kids oftentimes undermines their motivation. Rewarding every child, regardless of their performance, flattens the dopamine curve and lowers motivation for the activity.
High dopamine levels with no effort will deter motivation because it diminishes both the value of the prize and the value of effort required to achieve that thing. This is similar to how drugs like cocaine and amphetamines can be incredibly destructive. When people realize that they can experience intense pleasure without much effort, it becomes difficult for anything else to motivate them. Unearned dopamine releases can essentially transform humans into single-minded zombies.
Moreover, constantly rewarding every small positive action diminishes the significance of those actions and reduces one's ability to self-motivate, especially for kids. Intrinsic motivation is the strongest form of motivation, while extrinsic rewards can diminish the propensity to engage in an activity.