Batling SDave sends Batgirl this 1999 SI article on hating the Yankees. Batgirl found this point particularly resonant:

364. Rooting for the Yankees takes all the courage, imagination, conviction and baseball intelligence of Spam. It's like rooting for Brad Pitt to get the girl or for Bill Gates to hit Scratch 'n' Win. (This is why I'm proposing legislation that would allow only those born in one of the five New York boroughs to be Yankees fans. All others who root for the team will be considered overdog-loving, Eveready-chucking, bandwagon-hopping, fair-weather, brownnose, pucker-lipped human goiters and be required to turn in their pinstriped underwear or be tossed into the East River with only Chuck Knoblauch to throw them a life preserver.)