The US doesn't really go all crazy for their national teams in international sporting events like the worldcup (olympic games being a special case), even though "soccer" and the fifa world cup are steadily becoming more popular - or so I've read. You have your leagues and their internal championships like the world series (kind of preposterous to call it world series when it's only american baseball teams that take part..), Stanley cup etc. instead that are hugely popular - and high school/college sports and their competition for state/national championships.
When the german national football team plays, it represents the whole nation in a weird way - or so it feels. It becomes "us against them", and there are archenemies like Holland in football that one has to beat at all costs. It is a matter of national pride, even a form of patriotism, to do well. The hate relationship with Holland does not merge with other areas though, it's just true for football games where the national teams play. It is really alot of fun to go into a packed bar and watch the games there with others. I don't think baseball could ever generate the same atmosphere (I think watching paint dry is more interesting than watching a baseball game).
What you're referring to are auto corsos. They're slighly ridiculous and can jam up the whole city if you're unlucky. It is a very peculiar way of celebrating and I think mostly football fans do it. The scarves you saw had the colours of popular football clubs in Germany, blue and wite was probably Bayern München for example.
Italy is always a favourite, but so are Germany and many other teams.
I am curious how or if the football fever will grasp the US, it would be a shame to miss out on all the fun.