NFL - Patriots
I grew up in NE, so I am a Boston sports fan.
My family has always been Patriots fans, I grew up watching the games, and by that I mean I grew up watching them lose alot. The biggest party of my life was when they got their assed kick by the Bears in 85, I was 5 and I was hammered by the end of the game. Basically if I was not a Pats fan, my family would disown me.

NBA - Celtics
I spent most of my sports watching with my Uncle, and I grew up watching the Celtics, Bird, Mchale, etc.

As a backup I will cheer for the Suns, since my ex lived in Phoenix. I spent alot of time in that city and love it there. He was a huge suns fans.

NCAA Football - Boston College
Its the NE thing, they get alot of press, so I watched them alot as a kid.

NCAA Basketball - Providence College
I go to their rival URI, but I grew up going to PC games, whereas I have never been to a URI game.

MLB - Sox
I spent my childhood watching them lose, I remember when the lost in the ALCS during my first semseter of grad school, one of my prof said "We'll now we can go back to getting work done."

NHL - Bruins
I am not much of a hockey fan, but I grew up watching the Boston Bruins on tv, and going to the Providence Bruin games. Mostly though I like the fights.

Basically its all Boston, since that is were I grew up, going to Fenway, Boston Garden, etc. and watching Boston sports teams from a very young age. Some of my earliest memories are watching sports with my Uncle, and him explaining the rules to me.

I've been watching Boston sports teams for 2+ decades.

Edit:

Tennis: Federer and Venus Wiliams.

I always liked Venus better, I remember her first final against Hingis, with her beads in her (like I used to were mine), its just cool to see someone like me out their playing tennis.

Federer, I liked from that match were he beat Sampras at Wimbledon.

NBA - New York Knicks

Since '94-95. One of the first times we actually had NBA games live here, it it happened to be that Houston-New York finals. Way more appreciation around here was for Houston (a bit for the fact that Tomjanovich is a Croat), so I just had to select the other club. Lately I feel like shit after every game, but I do not regret it.

Football - Manchester United

I don't really remember how, or why I became of fan of them, was some 15 years ago. Was a big fan of english football, so guess I just picked one...who happens to win a lot. :-)

MLB -- Cleveland Indians
NBA -- Cleveland Cavaliers
NFL -- Cleveland Browns
NCAA -- OSU Buckeyes
NHL/Tennis/Soccer -- Don't follow

I was born and raised in the cleveland area and have been an avid fan of the cavs, tribe and browns since the mid 80's. I latched onto osu in the early 90's. i always wanted to go to college there but ended up in community college lol.

English Premiership: Arsenal
Followed them since I was like 5 years old, not sure why, first side that ever caught my attention on MOTD I guess, and they had the legendary back line of Adams, Bould and co when i began. Still support them to this day, plus they are entertaining these days which is a bonus

Scottish League: Celtic
Well I'm Irish, I think its some sort of law that if your Irish you support Celtic

Gaelic Football: Mayo
Gotta support the home county even though we haven't won the All-Ireland in over 50 years

Formula 1: Formerly Jordan
Until they got bought out and the Irish link went. Now I just follow the sport as a general fan, but I always like seeing Hamilton and Kimi doing well since they are honest racers...........and Alonso to lose since he's a git

Indy Racing League: Danica Patrick
I just want that breakthru for women in motorsport to be finally made and she looks the best up to do it.

Rally: Petter Solberg
Because the guy is hugely entertaining and comes across as a really friendly person to boot.

NFL: San Francisco 49ers.

I watched a football game on TV when I was 4 and the Niners won. Since then, they've been my favorite.

NBA: Los Angeles Lakers

Total front-runner thing. I've liked Kobe since he entered the league. Still, if he jumped ship, I'd still be a Lakers fan.

MLB: Boston Red Sox

By-product of a previous relationship. She was a Yankees fan. I couldn't stand for that and picked the team who was, at the time, a severe bitch.

College Football: Nebraska Cornhuskers

I live in Nebraska. That is all.

Baseball and football are the only professional sports I care about. So...

MLB- Seattle Mariners
Not quite the hometown team, but Seattle is only a two-hour drive away, so close enough. My parents started me on this at a young age (six) when we went to a Mariners/Rangers game in the Kingdome in July 1997. I went to the final game in the Kingdome in June 1999 (appropriately, against the Rangers) and the first game at Safeco Field three weeks later. My family has had season tickets at Safeco since 2000, so we're over there pretty much every other weekend from April to September. I was at their 116th win in 2001 (amazing year) as well as the All-Star Game (and all the festivities) that same year, the game where Ichiro got his record-breaking hit in 2004, and all the playoff games that have been played at Safeco (all of which were in 2000 and 2001). Love the Mariners. Hopefully they'll return the favor and win a World Series some time in the near future (the last year before I head off for college- which would be this one- would be nice).

NFL- Seattle Seahawks
Definitely the "hometown team" thing. The 2005 season was a lot of fun to watch, even though they lost the Super Bowl.