Perhaps it's because certain ie: a lot of people tie their identity to external stuff like music, movies, videogames you name it. So to criticize something they identify with directly criticizes a part of their very being. When one bases their sense of (group) belonging and derived pleasure from a certain external source, any criticism will directly come across as an attack and thus it's response is emotional based. And we all know how emotional mature some are when it comes to these types of situations.