Health, when it's good enough, is a good enough reason for anyone to want to have fun. Just because you feel good, you kick up your heels, jump for joy, leap for leaping's sake. And when you don't feel good, even if you're a kid, you drag your feet, hang your head, just don't feel like playing, and sometimes even bite. This is true in any culture, in any species where there is anything like play. Unless sick or threatened, what kids do is play.
Kids play almost all the time.
Dog kids and cat kids, goat kids and human kids. Especially the healthy kids. Sure, we do what we can to keep ourselves and each other safe. We have safe places and safe zones and, if somebody really gets hurt, we even stop the game. But, when we are truly at play, we don't think about safety.