Loyalty to ones team is an essential element of sport fandom, no matter the sport. It's half of what makes being a sports fan so much fun: ready-made villains to root against, and hockey fans know this all too well. Just ask a Montreal Canadiens fan how they feel about the Toronto Maple Leafs, or vice versa. Further, ask a Maple Leafs fan how they feel about the Boston Bruins. Same reaction (though arguable less curse-punctuated than it would be were you to mention Montreal, in fairness). Just look up any of the countries that have ever gone to war over soccer (seriously though) or any of the people working at a stadium that has erupted into a riot during a game. Team rivalries provide a group of "bad guys" to battle our heroes. This is an integral part of any narrative and as sports fans, we use that to place ourselves within the narrative. To live it and bring it to life. Being a fan wouldn't be the same without it.
Now any hockey fan knows who their teams rivals are and can tell you so, just as quickly as they'd likely be able to rhyme off the famous Original Six teams of the NHL. But did you know there were teams that existed before that lauded golden-age of hockey? I sure didn't. Thanks to the NHL, though, and this new time-lapse map of the league's evolution, we need no longer live in ignorance!