Well, the more a fanboy's favorite console sells, the more games said fanboy will have access to on said console. The more another console sells, the less the fanboy's does. If a fanboy's friends buy the same console as him, they can play together and/or lend each other games, controllers, etc.
The more a fanboy's favorite game sells, the more people play that game's multiplayer and the more sequels get made. The more another game for the same demographic sells, the less the fanboy's does.
Marketing is designed to do this to some extent. Fanboys read specialized press and tell their more casual friends about what's on the market. The obnoxiousness is an unfortunate by product.
Also, people like to look down on other people sometimes. Talking about how stupid it is to derive fun from something that the fanboy can't have fun with (EDIT: or afford when in addition to what they're already playing) is a way for some fanboys to do so.