On "Lolita" and the obligation of literature

On 16/8/2013 I wrote an entry called "3 wrong attitudes in reading novels" (avoid depressing books; like books with happy endings; have to like and sympathise with the main characters in order to appreciate the book).
These days going around the internet and reading several blogs, I realise that I forgot something else: the tendency, when looking at a book, to ask "What is the purpose of this book?" "What does the author want to say?" "What is the message?" "What is the moral lesson?" Well, why? Why does literature have to be moral and didactic? Why does literature have to carry the duty, the responsibility of creating good, sympathetic characters, i.e role models, and if depicting immoral ones, must criticise them and punish them? Cannot literature simply reflect reality and/ or be a case study in psychology? Cannot literature be enjoyed and appreciated for aesthetic value? Are books with moral lessons more worthy than those without explicit ones? What are immoral books (but those that show the world its own shame)? Why must the so-called immoral books be deemed worthless, or even disgusting, irrespective of their aesthetic value and the authors' genius? Why must novels be described, summed up in a couple of words about morality? Why are writers supposed to say something, teach something, make people learn something specific? Because otherwise people find no reason for reading fiction? Or because of the illusion that literature makes us better persons?
I am appalled by the people who refuse to read "Lolita" because it's wrong and, according to them, revolting. I am appalled by the people who read it and fail to see its beauty and wonder, or see it and then have a bad conscience for liking 'such a book'. There are 2 types of people who approach "Lolita" the wrong way- those in the 1st group mean to find steamy sex scenes and get disappointed; the others expect some explicit moral lessons and also get disappointed. Having such ideas and expectations in mind, they can't fully appreciate the humour, double entendres, word play, puns, anagrams, coinages, vivid descriptions, beautiful, poetic language and Nabokov's incredible ability to get into a paedophile's mind.
The more prigs I encounter, the more I admire Nabokov for writing and publishing "Lolita", for not stooping, for disregarding philistines, for contributing such a wondrous, magnificent work of art to American literature in particular and literature in general. 
Here is his afterword "On a book entitled 'Lolita'": https://docs.google.com/viewer?url=http://64.62.200.70/PERIODICAL/PDF/Encounter-1959apr/72-76/
And here is his "Playboy" interview, in which he discusses "Lolita" and literature: http://lib.ru/NABOKOW/Inter03.txt
Each time I see someone attack Nabokov on moral grounds, I picture him laughing. Think about it. Nobody calls "Lolita" a badly written book. Just 'immoral'.