Joy is one of the few emotions that has strong and powerful positive affects on us. It has to do with automatic and spontaneous evaluations and also with long term benefits. Joy motivates and energizes us to interact, to have fun, to achieve goals and to deep our social bonds. Playing is a good example as its generally envolves joy. Playing helps kids to develop skills, to understand and acknowledge their abilities.
The action tendencies embedded in joy is to reach out others and persist on doing something. We can also find ourselves in a more contemplative way, just being delighted with joy and pride, when we achieve an important goals in life.
Joy can be also a maladaptive emotion when there is too much of it as we can see it in people dealing with mania.