A Psychological and behavioral explanation
The nature of human behavior is, humans do things to satisfy their unmet needs in some way or the other. These needs may vary from the very basic such as hunger to reach the greater heights of life. Whatever it may be, when he realizes that some actions he takes lead to satisfy his needs, he tends to do them in a superior way, expecting the satisfaction of his needs in a greater manner. On the other hand, if a person perceives that the things he does are unable to take him to a state of satisfied on some need or want, and then it's obvious that he does not want to continue the same action.
An activity of any kind, a person is involved in takes that person to a result. The result may or may not be desirable. That depends on the activity and the way that particular person performs that activity. The activities done by a person and the way they are done can be collectively known as the behavior. Therefore, what leads to a result is the behavior. The needed results are led by the right behavior. The right behavior here is an identified set of activities and the way they should be performed, which at the end take the person to the needed result.
The two ideas: need-satisfying urge of humans' and behavior leads to results', forms the following idea. An individual may act in a certain way to satisfy his unmet needs, but that behavior brings a result, which may be satisfaction of that need itself or satisfaction of that need with something else. If that particular behavior brings some other result in addition to the need satisfaction itself, then it's obvious that encouraging that behavior by means of ensuring greater need satisfaction yields more results.
The above-discussed ideas make up the background for our topic, what gets rewarded gets done'. To get done something (i.e. to achieve a result) there has to be a right behavior. Rewards can satisfy some needs of an individual