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Because doing so makes you a brave and independent patrician proving that you are a higher form of life than the the plebeian fanboys, honks, homers and pom-pom waving rump swabs who have the audacity to root for the local teams. The fact that those four pro teams have won 9 world championships in 14 years is an inconsequential detail to be summarily dismissed. That type of smarmy attitude is exemplified by Shank Shaughnessy's continual, repeated assertions that the Patriots didn't win the SB on their own (Seattle gave it to them) and that the Red Sox winning the World Series in 2013 was a "fluke" (yeah, a 162 game regular season, going 11-5 in the postseason against Tampa Bay, Detroit and St. Louis was a "fluke.). They're all jerk-offs. But, as much as the contrarians hate the local teams, their greatest hatred and disdain is reserved for the fans those teams.