Gonna ignore pro sports for a second here -- I do think there's something to be said for civic pride but I'm the type who only roots for my hometown and I know that doesn't apply to everyone, and besides, the players are mercenaries anyway. It's not their hometown team, they just rent townhomes in nice suburbs 45 minutes away for a few months out of the year.
However ... college sports are different. If you attend or attended a university which has a football or basketball team win the national title, that means so much revenue and monetary value has been added to your school that it does in fact affect you. Out of state enrollment tends to soar at any given university following a national championship. Higher OOS enrollment = more revenue for the school, a larger endowment, higher national academic rankings, etc.
For better or worse, athletic success is a huge factor in how American college are perceived and valued. Five years ago i'm walking around NY trying to find a job with a degree from LSU, a pretty mediocre school from a pretty terrible region for education, in a region where I'm surrounded by graduates of much more reputable academic institutions. Of course, first thing my eventual boss says in the interview after glancing at my resume, "oh wow, great football team. I've heard the tailgating experience is crazy down there!" ... That leads to a 20 minute conversation. He's a Notre Dame fan it turns out. He likes college football. He has only heard of my school from college football. But hey, common ground.
... He never got around to asking me about the school's broadcasting program. Because who gives a shit? As sad as it may be, the value of my education is directly tied to the fortunes of the football team.
So yeah, when my school wins, I'm not celebrating because the guys wearing the right colored laundry won. I'm celebrating because all the money and resources that have been dumped into this pursuit by myself and fellow students/alumni/faculty/players/coaches/staff etc. has paid off, and will yield additional money, resources and recognition down the road.
As for the rest of you bandwagon fucks cheering for a school you didn't attend, meh. Find a hobby. Losers.