In terms of Boston sports, my wife moved to Boston with no sports allegiances of any kind. We went to a few games at Fenway, but she found the fans obnoxious at best, and passively racist at worst, and we stopped going. We went to minor league games instead. Pawtucket was great, but New Hampshire was as bad as Fenway, and this was AA baseball.
We didn't go to any Celtics games while she was there, but my one Celtics game in 2002 left me with a similar bad taste (except the fans were blatantly homophobic instead of passively racist). Despite them being my favorite team, and living in Boston for another decade, I never returned to the Garden (or Fleet Center/whatever name it went by.)
We moved to DC in 2012. Nats fans are much nicer than Sox fans. I can show up to a Wizards game in a Celtics shirt, and no one says anything, except maybe some playful banter. Minor-league baseball is as family-friendly as it should be, unlike we found it to be at the games we went to in New Hampshire.
Maybe for some people there is a history that goes back decades, but our exposure to and knowledge of Boston fandom is largely from the last decade or so, and even though I'm a fan of the Celtics, my opinion of Boston fans isn't that far off from those who root for rival teams.
Mind you, I don't think it's all fans, and it's probably not a majority of fans, but there are enough to create a very off-putting atmosphere in a way that we haven't experienced elsewhere. (We've also been to MLB games together in Pittsburgh, Baltimore, and the Mets. We haven't seen the Yankees because we imagine it would be similar to Boston.)
And that's not to insult Boston. We'd move back tomorrow if we could. But it is real.