Mostly boring with one interesting one:
Grew up in Buffalo, so Bills and Sabres
Went to Penn State, so the Nittany Lions
Travel soccer team growing up was the Arsenal, so root for them.
The UNC basketball fandom started in HS. I have a brother a year younger than me and we were pretty competitve. My freshman year, his 8th grade year, our Varsity High team played against a team from downtown Buffalo so we went to watch the game. The Buffalo team crushed us and one player was really good. My brother said "I'm going to find out where that guy is going to college and start rooting for them". We saw in the paper the next day (yes, I'm old) that he was going to Duke. So my brother started rooting for Duke and since I am a good brother, I started rooting for UNC. We had tons of fun through HS, college and post college going back and forth during those awesome rivalry games.