I posit that the obsessions of young girls with male relationships in fiction and celebrity culture has to do with (naturally) the relationship of the young girls to masculinity in general.
Going through puberty and growing up female is a minefield of expectations and dangers. Little girls quickly learn that they must cover up their innocent bodies or risk censure, harassment, and even bodily danger. They learn that they are not expected to be full humans, much of the time, but accessories or bit parts in the drama of the lives of men. Other people have remarked on the possibility that boy bands give girls a chance to experience romantic feelings without the pressures and dangers of real life relationships with real boys, but I would also put forward that the infamous ‘fangirl’ obsession with certain kinds of close male friendships and male relationships in general so idolised by young women stems from a similar root. In their fantasies, girls want to believe in the humanity of the male characters they love – and the humanity of boys and men in general. There is much justly and truly said about the objectification and dehumanisation of women in sexist pop culture and society in general – but, less remarked upon is the dehumanisation of the male. When girls are taught – for their own safety – to fear and mistrust the actions and motivations of the men and boys they encounter (“men only want one thing”, etc), an enormous, dehumanising distance is placed between girls and boys – not only from the position of patriarchal power, in which girls are dehumanised in the perspective of boys into objects for use and pleasure, but from the perspective of girls, in which boys are dehumanised into animalistic creatures without empathy, vulnerability or any inner life. Of course, both of these perspectives are harmfully absorbed by the targets also, in an endless feed-back loop of damage and suffering: girls see themselves as objects with only material value and boys deny and repress their own emotional and spiritual growth.
Girls fetishise and idolise male fictional characters because the medium of fiction gives a girl free, unrestrained access to the inner lives of masculine people – not because they wish to feel that the inner lives of men and boys are so mysterious, but because they are not: because girls need to feel that emotion and vulnerability are common to all people, something that patriarchal macho culture plays down and outright denies. In experiencing the inner life of a fictional male character, a girl is safe and free from all possibilities of deception, betrayal and abuse, from which she is so strenuously taught to shield herself and fear.
And girls like to vicariously experience these stories within male-male relationships, usually relationships that are written as straight friendships between characters who are not romantically involved. This cannot be too surprising if we look at the alternative – it is incredibly difficult to see oneself in and project oneself onto most of the female characters portrayed in popular culture, opposite the most compelling and interesting male characters. In Sherlock, for instance, it might be possible to sympathise with the ridiculously sexualised and emotionally paper-thin Irene Adler, who merely exists to provide endless displays of female flesh for the camera to linger over... but how much more easily we could instead project ourselves onto the loyal, brave, human John Watson, instead, who actually has a close, meaningful relationship with the title character! Irene Adler doesn’t have the time to really form an enviable emotional tie to Sherlock – and she’s not particularly designed to, either.
I won’t touch on ‘shipping’ culture (in which fans of a public figure or fictional character invest immense amounts of emotional energy into said person’s possible romantic relationship with another character/characters), because I personally don’t engage in this practice, but I would be very interested to hear a shipper’s perspective on this theory?