From my experience, it seems there are two types of sports fans.
The first type supports their local team by default. It is the team their father followed and it is the team their brother has season tickets for, so the allegiance thus comes almost by birthright.
Then you have the other sort: the fan who comes to a sport late or who never much cared for any particular sports outfit for whatever reason.
As the Premier League continues its progress into the American consciousness, it is pretty clear that most new fans are going to be looking for a rooting interest.
Manchester City fans like myself have myriad arguments for backing the Sky Blues.
But more than anything else, friends don't let friends support Manchester United.