I think it's basic territorial nature - and seems somewhat biological. I think, especially young people, whom most of the fanboys seem to be, are wired to support their 'tribe.' Right or wrong unity for some surpasses most all other needs. They do feel that their tribe is threatened if a complement goes to the 'other side.'
It the same way I've always thought it weird how people will love some sports team - then move do a different city and switch sides. And if they don't play or participate in the sport, I don't understand why they get so crazy about it.
Anyway, I think it seems to mellow with age. People may still have their first love, but aren't so threaded.
Oh, an I get to be and 'all around' gamer!