You hate New York sports team.
Starting from a young age you learn that part of being a fan of all the Boston sports teams (Red Sox, Patriots, Celtics, Bruins, etc) means that you hate all New York teams (Yankees, Giants, Jets, and so on). Some of the biggest games are the rivalry games among those group of teams.
You get into verbal arguments with Boston sports haters.
When people say anything negative about those teams you get a little heated. You let those individuals know just how wrong they are and even try to bring them over to your side. P.S. Deflategate never happened and Brady will forever be the greatest of all time.
Losses are painful.
When any of your teams happen to lose the next day sucks. If anyone asks about those games you give them the look of defeat as if you were one of the people playing in the game. The teams have become part of your life and will always hold a special place in your heart.
You're used to your teams winning.
Boston sports fans are spoiled in that the teams they route for tend to be good. If there is ever a losing season or season where one of the teams seems to not be doing so hot it's unusual.
If you're not with us, you're against us.
It seems as if everyone who doesn't support our teams is routing for the failure of us. They are just jealous that they're teams are underperforming and getting killed by ours. They are bitter individuals.
We're like one big happy family of sports lovers.
When you find other people who love the Boston teams as much as you do, there is an instant connection. You can talk smack about the Yankees, the Giants, or any of the other teams and you know they will have the same opinion as you.
And you can't imagine being a fan of any other group of teams.
Let's be honest, you want to route for the best so Boston teams are the obvious choice. They tend to win and when they don't it seems like a bad dream.