One of the many extraordinary things about the Harry Potter series is the way in which the celibate and unrequited love of Severus Snape for Harry's mother Lily all-but usurps the main plot - and forms (for a sizable minority of fans) the centre of the whole series.
In other words, for some fans, Snape - rather than Harry - is the most interesting and sympathetic character.
This was clearly unplanned by JK Rowling - who has stated that she regards Snape as a nasty character redeemed only by his love (for Lily) and the courage which this inspires - but otherwise very deeply flawed - almost to the point of sadism.
It is a remarkable thing in a modern context that Snape should take on this role; especially as he is portrayed in the books (big nose, greasy hair, awkward loner).
Of course, the casting of Alan Rickman in the movies may have something to do with this, since Rickman's screen persona is intrinsically attractive to women of all ages, regardless of his role.
That aside; I see this phenomenon of Snape-mania as yet further evidence of the benign subterranean influence of the Harry Potter series - that Rowling has made the permanently unrequited love of an unattractive social reject (albeit one of great magical powers) for a 'popular' beauty into a morally-admirable thing for tens of millions of readers.
This fact is only understandable in terms of the other-worldly backstory of Harry Potter - the immortality of the soul, and life beyond death.
Snape is indeed brave in life - but from a this-worldly perspective his unyielding love for Lily can only be seen as a pitiful delusion. The fantasy life of a 'sad' man.
Yet that is not how Harry Potter readers experience it: they see Snape's love as being - not requited - but validated (or redeemed) beyond the grave.
This in the context of the biggest publishing success of the past generation!
I say it again: remarkable...