Soccer is a lot more interesting live than on TV. The field is too big for TV. You can't see plays develop at all. Plus, half of the reason to go to a soccer match is for the experience. The fans tend to be super zealous, singing songs, with great rivalries, great characters, etc. It reminds me of the way my in-laws are died in the wool Boston Red Sox fans and hate the Yankees to the death. Unless you're from there, to see it with your own eyes, you can't understand it. I know some soccer fans have gotten violent at times, and that's too bad, but overall the entire atmosphere at matches is terrific.
It also helps the more you understand the sport. The same way many cycling fans see a bunch of riders along the road and don't know what's what, the more you understand strategies and rules in soccer, the more interesting it is.
I'm not nationalistic as well. If the US wins, I'm happy, sure. But I find myself the same in other sports, cheering for underdogs. And like baseball and the Yankees always winning with all their money, or basketball where every other year it's the Celtics and Lakers (though that's probably rigged by the refs and league) I find myself rooting against the favored teams who have won many times already, regardless of the country.