I moved around a lot as a kid but spent most of my formative years (all of high school and some middle school and some elementary school) near Boston rooting for the Red Sox and the Celtics and the Bruins and the always hapless Patriots. This was spanning late 80's to 1999. So the Celtics had some dominance and the Pats flirted with being OK towards the end with Bledsoe and Parcells, and the Red Sox had some great players but continued the painful losing tradition year after year. They all stuck with me after I left Massachusetts. Boggs, Lynn, Rice, Nomar, Pedro, Vaughn...the Sox had some great players that made them fun to root for. This was also the era of the dominant Yankee dynasty, coincidentally. The Yankees hatred was a unique bonding thing and Sox games vs. the Yankees reminded me of football matches I went to in the UK as a kid in their intensity.
My entire family are 49ers and A's fans as we have Bay Area roots. (I was actually born here. As a very young kid in the Bay Area in the 80's I rooted for the family team--the A's. I can still name the Bash Brothers line up by heart.) The Giants are my "girlfriend" team on the side. I can root for them and celebrate with my fellow San Franciscans as there is rarely ever any conflict with rooting for the Red Sox. My nightmare scenario is a Red Sox v. Giants World Series. I'd have to go for the Sox but it would be bittersweet.