Ughh...
10 years ago, this city hadn't seen a championship in 13 years. 6 championships later, things have obviously changed.
EVERY city has fair weather fans and bandwagoners. This isn't a problem unique to the BOSTON sports fan. I'm sorry, you see it in any city. You also see some loud, arrogant, and incessant fans. That comes with the territory. Championships make these people even more insufferable. The advent of the internet, sports radio, and all sports cable networks has provided more avenues for these very vocal fans to brag and vent.
Once a team wins a championship, you have a lot of people wanting a piece of the action. After the first championship, you want to see your team comeback and down it again...establishing a legacy. You want your team to go on a historic run and be mentioned in the same breath as the '80s Niners, '80s Celtics, or '90s Yankees; not some one and down championship team. This "thirst" for more makes many fans more critical and more "interwined" with their teams. Thus, this leads to the complaining after losses and second guessing. That can also rub people the wrong way.
The thing I love about this city is that there is passion for their sports teams. When one of these teams makes a championship round, the city/area unites and that provides for some special moments. You can walk down the street at lunch break and see guys in business suits rockin' Sox hats, sports bars are mobbed on Tuesday night for a game, people driving around honking and waving flags. I love it. These teams are very much a part of our conscience here in Boston. Not every city can say that.
Some other city will go on a run the next decade and I'm sure that you'll see "I hate (fill in the blank) fans".