It isn't so much why you become a fan of a team, because fandom can be fleeting. It's why you stay a fan that matters. I've been a soccer player and fan since going to an NASL game in 1978, and US teams and leagues came and went, rose and fell. I was an early subscriber to Soccer America magazine, and for years I devoured the box scores and standings of far-flung leagues around the world. I played in the ethnic leagues of Southern California and in pickup games in Sweden, Spain, and Portugal, attended and watched club and national team games in those countries and in Mexico. But I never gave my heart away.
When Major League Soccer launched in 1996, my wife and I were living in Washington state – now a soccer hotbed, but then home to no first division teams. For whatever reason, I believed this league would survive and grow into something permanent, and I committed as a fan, not to a team but rather to the league. With a more widely available television package and the emergent web, it was possible to form communities across the country and the world, and by 2000 I was participating in online play-by-play conversations in a weekly MLS thread on BigSoccer.com. The league as a whole was mine.
Then in 2004, a franchise was awarded to Salt Lake City. That may not seem close by, but it would be possible to get there in a day’s drive at "only" 650 miles away. I went twice during the first season of 2005 and bought a shirt, and followed the minutiae of roster building and stadium negotiations and building of a fan base online and on television. I am not sure I was fully committed, but as bad as that team was (and it was an abysmal season) I enjoyed having a team to root for after almost thirty years.
In late June 2005, Real Salt Lake sent a squad to play an exhibition game against the Spokane Shadow; I travelled alone to the game. Almost 3000 people were at Joe Albi Stadium that evening, all there to root for the Shadow except for me. The teams were stretching and knocking the ball around, and Salt Lake player Jamie Watson looked up and saw my claret RSL shirt amongst the white and blue clad pro-Shadow crowd. He nudged/caught the attention of a couple more players, and they turned and gave me an ovation as I walked across the front of the stands. I was blown away: pro athletes from the team I rooted for were clapping for me specifically, not just saluting their fans generally. And that was it; the bond was formed because it was a two-way thing.
When the Sounders, Timbers, and Whitecaps later joined the league, there was no chance of me shifting allegiance to one of those much-closer teams; Real Salt Lake had chosen me as much as I chose them, and I will stay a fan for the long haul.