I'm sure none of this is original, but what ever is? I think we enjoy games because we innately desire to compete. There is a drive within us that wants to test our own capabilities, see how we will perform in a tense situation. I can even propose that we are trying to determine how we will face the final test: the moment of our death. (For further reading pick up the Dune series, specifically God Emperor of Dune.) The output for these drives expresses itself in other ways, often violent ones such as waging war, but our society suppresses and redirects these drives to promote our own survival. We can see the violence in a number of the games and contests, though. There are any number of "contact sports" such as American football, rugby, etc. which show obvious battle overtones. Going even further, we see wrestling, boxing, and mixed martial arts showing continued popularity, and not just for spectators. There seems to be no shortage of volunteers for these activities, willing to put their bodies and even lives on the line.
Not all sports are violent, but most involve competition. Teams face off in basketball games, two tennis players battle for a win, sprinters try to outrun their competitors. Even in such athletic pursuits as we see in track and field events, like pole vaults, long jumps, and discus throws, the competition exists however indirectly. The participants still push themselves to be better than others who do the same. Of course, not all of us have the gift for athletic pursuits, but we find our own games. Card games (poker, gin, hearts) have long since been a competitive pursuit, and we can all recall any number of board games we've enjoyed. Interestingly, it is here that we see the roots coming out again. Risk, Axis and Allies, and any number of others are wargames, allowing us to try our hand at conquering the world, if not ruling it. Wargaming becomes even more detailed and allows even deeper strategies in such games as Chainmail and Warhammer. From these roots then evolved my own particular hobby, role-playing games such as Dungeons & Dragons. RPGs are rather interesting, in that the people you play the game with are not your opponents, but almost always teammates. Even the person running the game (be they DM, GM, Storyteller, or what have you) is more a guide and adjudicator than an adversary. Its a unique blend of competitive game and social gathering that I particularly enjoy.
Computers have ushered in a new era in playing games, since now we have a choice between facing a human or a computer-controlled opponent. RTS games let us work through battle tactics without turns, reacting as the combat flows. We slip into the roles of various characters in any number of first-person shooters, RPGs, and action titles. Racing in cars, boats, planes, on snowboards, skateboards; all of these are within our grasp and avoid all the old possibilities of injury. Soon, we'll even be able to play our good old D&D over the Internet with D&D Insider. There are those who worry about this medium of entertainment and competition, concerned that by simulating violent or dangerous situations, we'll slip into them in "real life." Personally, the pictures I can simulate in my head when reading a good book, or for that matter when I played pretend as a young child, are far more vivid than anything we can put on a computer screen. We've been simulating war and entertaining ourselves with violence for centuries, but some people don't want to admit the ancient drives that we're letting loose get their outlet in the Super Bowl.