In the May 24, 2008 issue of New Scientist, there is an article that discusses which characteristics truly distinguish human beings from other members of the animal kingdom: So You Think Humans Are Unique? (subscription required but you can get more information about this subject here). Culture, "mind-reading" (the ability to outwit/deceive someone else), emotions, tool use, morality and personality are all traits that were once thought to be uniquely human but have now been discovered to exist in varying degrees in members of other species. The article concludes by asking plaintively, "What's left to us?" The answer to that question is art, cooking, religion, humor--and sport: "All social animals play, but sport is a unique kind of play often entailing special equipment, complex rules, referees and dedicated spectators. Far from being trivial, sport is underpinned by many of our most advanced cognitive abilities."
So the next time someone asks you why you spend so much time playing, watching, writing and/or thinking about sports, there is your answer!