Joyfulness is a state of being extremely happy. The joyfulness of a bunch of little kids in a bouncy castle is hard to beat.
Anything that gives you a sense of deep pleasure or excited happiness produces joyfulness, from being surprised by an old friend you haven't seen in years to watching your favorite team finally win a big competition. When you're experiencing true joyfulness, you can't help but smile. We can trace joyfulness back to a root that means "to rejoice."