Kickball is so much fun because, to be frank, there is really not that much skill involved. Since you probably played it as a kid, I'm sure you remember all the rules - it is similar to baseball.
You had so much fun playing when you were in elementary school, but when you play as an adult; it's even more fun because you feel you are back in elementary school. Plus, all that running around helps release stress, gives your brain a break, works your heart and your muscles too.
