Thanks for your answer. We see this very differently. To me, it makes perfect sense that someone would like Celts/Yankees. It's two dominant franchises that have spent decades at a time being ahead of the curve, and that have been willing to do almost anything to win. And, in general, I don't see why you'd be pinning your fandom on team location. Do you cheer only for Boston area golfers and tennis players?
I'm a Red Sox fan because I was such a big fan of the Rice/Lynn/Evans outfield, and hated Reggie Jackson so much, when I was forming my allegiances. I'm a Bruins fan because of the greatness of Bobby Orr, Fred Cusick and Johnny Pierson, the existence of channel 38, Providence College hockey camp, and mini one-on-one. I'm a Celtics fan because of, well, pretty much everything prior to the two player deaths (Bias, Lewis). Because I didn't start following football as early as I started following the other sports, I'm a Patriots fan mostly because the Oilers weren't on TV enough during the Earl Campbell years, and "Favorite player" lost out to "local team that's easy to follow and has a field that's a great place to tailgate". But, if the Oilers had been in New York, I'd probably have ended up as a lifelong Oilers fan.