This happens with ANY city that ends up with a long-term dominant sports franchise. I remember when I was in High School the Cowboys were everypne's favorite team all the sudden during the Aikman/Smith/Irvin Era. After a while everyone started turning on the team because they were sick of how Cocky Dallas fans were getting and sick of seeing the same team dominate every year.
For me...Back before the Pats won it all, it was really awesome to experience 2001 and ALOT of people were jumping on the bandwagon and rooting for the Pats to finish the job and beat the diminant Rams. They were the most popular team in football all over the country because of how they won, and the fact that they had not won before. It was great being a fan and watching them, and seeing all the national support they got. Then, over the next few years they kept winning, and winning. It was still awesome for us Pats fans, but the rest of the country was sick of it. Added was the Red Sox championship and that just tipped the scales and made alot of people hate Boston fans and teams because yes, we got cocky. It's not a bad thing to get cocky and expect your favorite teams to win while they are on dominant streaks like the Pats, Sox, and now hopefully Celtics are on. At the same time the ire drawn from other teams' fans makes it even sweeter, for me anyways. I love the fact that other fans are angered by my favorite teams winning alot. It all depends on the situations. It's not like Boston fans care what other fans think of their teams anymore.