Unless it's Bears-Packers, Michigan-Ohio State, or Yankees-Red Sox, rivalries seem to come and go with regularity - in virtually every sport.
Of course, the Phillies' division rivals are the Mets, Braves, Nationals and Marlins. But depending on what year it is, the Phils' primary rival has essentially fluctuated between the Mets and Braves for the last 20 years. For a time, there was even talk that the Dodgers, who met the Phillies in the NLCS in both '08 and '09, could be in the mix. You could even argue that their biggest rival at the moment is the Giants. But as we take a look to the future, who do you see the Phillies' primary rival being in, say, 2013? What about 2016? The Nationals are coming on strong in terms of young talent, but the Braves have the combination of youth and experience that could give them more staying power. The Marlins seem to always be on the verge of breaking out, but it hasn't happened yet. And the Mets, well, it's nice of the Phillies to let them play baseball with them 18 times a year.