Caution: Old man ramblings ahead....proceed with caution!
With all due respect I can't completely agree with your statement that it's not a lifetime commitment you are born into. At least give those of us that were "born into" being a fan of Boston sports teams some credit for remaining loyal to our teams over the decades despite our current gegraphical location and years of poor performance regarding essentially every one of those teams over these years.
For some of us, and really I'm only including myself in this but I think I can safely assume I'm not the only one, it's a tie to our roots.
I grew up in Plymouth, Mass and remember Boston sports as some of my first memories.
I remember going to my Uncle Jack's house in Arlington with a Sox game playing on the radio, it couldn't have been any later than 1971 because we originally moved down to Florida in 1972 and my Uncle passed away while we were gone but what I do remember of this was my Uncle laughing and trying to get me to spell Yastrzemski, I think I managed to do it finally and still think that was a pretty impressive feat for a 3 year old.
We moved back in 1973, first memory was my Dad who was an avid hockey fan got me this cheesy inflateable bear with a #4 on it, the last name Orr was also a lot easier to spell than Yastrzemski and I loved going to the Garden on rare occasions when he was feeling well enough to take me to a game.
I remember having a red and blue stocking cap with a little Pat Patriot patch on the front of it that I'm sure kept my ears from freezing off during the Blizzard of 78.
I remember being very much surprised while watching a Celtics game one time of my Mom coming in to watch with me and telling me that her Father used to take her to Celtics games and that as a girl growing up in Roxbury she herself played basketball!
These teams are professional sports teams, it's true but it's more than that to some of us. It's a link to the past, to family members and friends long gone. I am now living again in Florida, I've lived in Indiana also but in my heart I will always be a New Englander.
I really do understand the excitement of following a winning team, I've felt that excitement now with the Red Sox, the Celtics, the Bruins and naturally our beloved Pats but I feel I've earned it more than someone who sheds their colors during the lean years.
Loyalty matters.
P.S. I hope this won't be taken as a slam to anyone with different feelings on the subject, only one fans perspective.