You ever see two puppies or kitties wrestling?
They do it naturally, and they have a high old time at it (at least until somebody bites or scratches a little too hard).
Why do they do that?
I mean: Why are they programmed to engage in fun, mock fights when they are young?
Because they're going to have to engage in real fights when they're grown. The rough-and-tumble play instinct lets them practice in a safe way what they will have to do in earnest later on. It exposes them to situations that are like real fights so that they can get accustomed to them, but without the danger that a real fight has.
That's why this playful wrestling is fun. It's to get the creatures to do it (they get the reward of fun) while they're simultaneously learning about something that will not be so fun later on.
And humans have the same instinct, which is one of the reasons human boys wrestle and engage in rough-and-tumble play and play cowboys and indians or cops and robbers or whatever the local cultural variant of the game is.
It's also part of why we get a thrill out of reading suspenseful or scary stories: We mentally put ourselves through dangerous situations in such stories (vicariously, through the characters) so that we'll be able to better handle danger
ifwhen we encounter it in real life.
This kind of play thus has an important function.
But what happens if you get a society full of parents that are overprotective of their kids and who think that is the duty of parents to completely shield children from risk rather than helping them learn how to accept and manage risk?
MORE THAN YOU MIGHT THINK.