MLB - Cubs - I was born and raised in Chicago so I have too many memories of going to Cubs games with my family, watching games on WGN on summer afternoons listening to Harry, and living in Wrigleyville after college. That and you can’t beat Wrigley as the greatest park in baseball.
NFL - Seahawks - I live in Seattle now, and when we moved out here, you could only catch one or maybe two Bears games a year on TV. I started following the Seahawks so I could follow a team every Sunday. And to be fair, I was rooting for the Seahawks back when they were still self-destructive, frustrating bastards… though their current success is cool. It does make for interesting games the last couple of seasons since I still have family in Chicago that are die-hard Bears fans.
FAPL - Bolton Wanderers - Don’t know really… they were just “assigned” to me to when I was working in London, and all the guys I worked with supported all the good teams (Manchester United, Liverpool, Chelsea, etc.). So, the British guys said… “here, support these guys.”