I have only ever cheered for the Red Sox once in my life. That was when I first got into baseball, back in '07. I was pissed at the Indians for beating the Yankees in the ALDS and I felt that I couldn't turn around and, in the ALCS, root FOR a team that I had just been rooting against.
I didn't realize the true extent of the Yankees/Red Sox rivalry back then (I hardly knew there WAS one, in fact). I won't make the same mistake twice.
(Note that even though I hate the Red Sox, I like Boston itself very much and I love Fenway Park.)