Play:

>play is comprised of sequences in which the players switch rapidly between well-controlled movements similar to those used in "serious" behavior and self-handicapping movements that result in temporary loss of control. We propose that this playful switching between in-control and out-of-control elements is cognitively demanding, setting phylogenetic and ontogenetic constraints on play, and is underlain by neuroendocrinological responses that produce a complex emotional state known as "having fun."<

Marek Spinka et al. - Mammalian Play: Training for the Unexpected

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"According to our hypothesis, play enables animals to develop emotional flexibility by rehearsing the emotional aspect of being surprised or temporarily disorientated or disabled. Although unexpected events that occur in a dangerous situation would likely magnify fear in inexperienced animals, we suggest that fear is modulated in play by the relatively safe context in which play occurs and the improbability that losing control will have serious consequences. In addition, regaining control following an unexpected challenge is likely to be rewarding, and the positive nature of this experience may be intensified by the rapid repetition of in-control and out-of-control elements that occur in play."