I don't know about anyone else, but I grew to love my favorite teams (Packers, Yankees, Celtics) because they were favorites of people I grew up with.
The first sport I ever really watched a lot was baseball. My grandmother was a huge Yankees fan and I always used to watch the games with her. Many moons later I still find myself cheering the Yankees.
Basketball was next and I always heard my mother talking about Larry Bird and the Celtics. Again, I find myself cheering the Celtics through all these years.
How many times have you watched the Packers on thanksgiving day? Now, how many times have you celebrated thanksgiving with your relatives? My uncle was a big Packers fan and my mom and grandmother were Eagles fans ... and ... well ... I guess it's because I don't like birds. I chose the Packers.
I also realized that all of my favorite teams hold the records for most title wins in their respective sports. (I realized this after I became a fan, mind you. :-x)
I really don't support my local teams very much. The Eagles I HATE with a passion. The Steelers ... well, I can tolerate them. The Sixers I can watch and sometimes find myself rooting for. (Unless they're playing the Celtics. ;-)) And the Phillies and the Pirates ... are the Phillies and the Pirates.
And I hate hockey.