Familiarity Breeds Contempt, Pt. 1
This next bit borrows (by which I mean steals) heavily from two books, in this order: Raph Koster's A Theory of Fun and Laurence Gonzales' Deep Survival, both of which discuss the root of the human urge to engage in challenging (or even risky) fun.
Complex social animals like tigers, dolphins and humanfolk spend fewer of their formative months in the womb, so they emerge into the world less able to fend for themselves than, say, fish or lizards or bats. On the one hand, our young come out without any special wiring that lets them immediately stalk prey or flee predators or engage in complicated locomotion. On the other hand, our young learn how to do all those things in a fundamentally different way, taking advantage of their more sophisticated brains to construct progressively more detailed and accurate models of how the world works and how they can optimize their time inside it (however brutal and short that time might be).
And this is where play comes in.
