Every fan has a team they like to cheer for. Maybe you're not a die hard fan who bleeds the team colors and likes to cheer from the comfort of your own home, but everyone who has a passion in sports has a team or player they like.
Conversely, most people tend to have a team or player they can't stand. You know when your favorite team plays their rival it's not simply "just another game." The emotions are at an all-time high and it seems like every play or moment that ticks by carries extreme importance.
The same holds true in the fight game. There are certain guys that evoke emotion out of fans, whether be it love or hate. It's hard for anyone to not cheer for guys like Brian Stann or Junior dos Santos, while it's very easy to jeer guys like Michael Bisping and Alistair Overeem.
Rivalries are what help shape the culture of a sport and they've been crucial in the development of MMA into a mainstream sport. Some of the sport's biggest rivalries have become some of the biggest stories and helped to catapult MMA to new levels.
As important as they are, it seems like MMA has phased out the rivalry aspect of the sport. Now it's "just business" to hype a fight. If MMA is to continue to grow, rivalries will need to stay an integral part of the sport.