I watch soccer about the same as curling. It comes on once every four years and I root for the USA.
What's so interesting about soccer. I don't think there's enough action to keep most Americans interested. A typical score is 1-0.
It takes a great collective effort to score a goal most of the time. It's not like the NBA where one man can take you pretty much all the way to the finals (like Iverson's Sixers, LeBron's Cavs, Dwight's Magic, etc.). Messi might be the world's greatest soccer player, but he wouldn't be half of what he is without Xavi and Iniesta in Barcelona. The excitement is not just about the sport itself but the environment, the supporters, the love for your team. We (and Argentinians as well, and some other parts of the world) have another way to interpret soccer, we feel it like a lifestyle. We go to every stadium, we travel to other countries to support our team, we sing during the entire match, we fight the rival supporters, our mood depends on our team's results.
Most of you might see that sort of fandom as idiotic, but it's just the way we feel about soccer. Winning the World Cup is truly being "a World Champion", not winning the NBA or whatever league you have there.