Well, I can understand changing from being a Phillies fan to a Yankees' fan. I am from Philadelphia, but despise all their teams (and most fans) except for the Flyers (where I was an usherette as a teenager). But as someone who know considers herself from Boston, I could never accept a significant other who was a Yankees fan . These are cultural differences that affect the whole world.
When I moved to Boston, I went to Red Sox games constantly. I was a huge Glenn Hoffman fan (#18 shortstop 80s and 90s and more recently considered for manager), but as long as I lived in the Boston area I could go to games. In Maine I can't, and so my primary love is after the Flyers ('67 starting being a fan) and huge [alas defunct] Montréal Expos fan (saw games in both Jarry Parc and Olympic stadium), I spend all my time with the Patriots, whom I've loved since I moved from Philly to New England in 1976. Those were the bad ole days of the Patriots, so I am no fair weather fan. I've been through multiple stadiums, horrible QBs, and then -- one fine day in NE sitting in the house of my parish manager -- priest was there too -- Drew Bledsoe got injured... And to the horror of them all I cheered!!! Because Drew Bledsoe, while he later showed himself to be quite classy -- did nothing for the Pats. They brought in the last NFL draft choice of a certain year, a little known kid who was known more or less as the ball boy who got water etc for the 'real' players.
His name was Tom Brady.
(Foul and Fairweather Fan)