Clearly, there could well be physical and social consequences of being too happy or having too much fun. Usually, activities such as self-handicapping and role-reversing work to keep play in check and restrain individuals from violating rules of the game. Play signals such as the "bow" are also used to initiate social play and to punctuate and carefully negotiate ongoing and often frantic, vigorous, and highly contagious play interactions. Indeed, play rarely escalates into full-blown aggression or harm because individuals play too hard with one another. Animals work hard to maintain fair play and fair play may be related to individual reproductive fitness (see also). Nonetheless, young animals in particular engage in vigorous social and locomotor self-play during which losing oneself in the activity can be detrimental.
Stabilizing selection and the evolution of happiness and fun
These questions lead to the general questions, "Does natural selection actually work to curtail limits of happiness and fun? Are there costs to being too happy or having too much fun?" Of course, part of growing up and becoming a card-carrying member of one's species involves taking risks, but can being too happy or having too much fun actually incur significant and long-lasting costs? We really don't know.