Too often we use our hobbies as an excuse to exclude others. If someone can't speak "leet" or doesn't know the batting order of the most recent World Series champions, we consider them unworthy. Everyone needs to feel important, but using hobbies to exclude others leaves us unhappy in the long run. No two hobbies better illustrate this than gaming and sports. The similarities between the two interests are staggering, and yet fans of one will often exclude fans of the other.
Retelling the stories of past experiences, complete with sound effects and swooping hands, is an exciting and powerful part of both hobbies. Reliving those moments with someone else not only allows you to enjoy the emotions of the moment but to form bonds connected to the memory. I remember acting out previous missions from Falcon 4.0 with my GM just as fondly as I remember sharing the 2006 St. Louis Cardinals' World Series victory with a neighbor.
Very few of us can be barbarian warriors or all-star power forwards - I personally want to be an all-star barbarian power forward warrior - but we can all enjoy being a part of something. Gaming and sports engage the mind and imagination and allow us to partake in a communal experience, whether it's through your guild membership or the logo on your shirt.
Part of what gamers enjoy is the rule sets and statistics inherent in gaming systems. Gamers often invest huge amounts of time to learn the minutiae of the games they enjoy. I've read rulebooks of games I have no intention of playing and then argued their merits with others. This is similar to the sports fans that learn the rules and trivia for their favorites, like the precise batting order of World Series teams.
With so many things in common, why is it so hard for gamers and sports fans to be friends? I suspect it's rooted in our childhood. When we're young, we tend to gravitate toward people with similar interests and don't develop the skills to interact with people different than us until we get older. So we evolve into primitive groups, like "geeks" and "jocks." For a lot of us, the groups we join at an early age are so rigidly defined we tend to conform to them even when we're older, and the stereotypes we form about other groups when we're young often stick around.