I think the OP did not only ask questions about the nature of the connection between Sandor and Sansa. He as well wanted to know where the fascination of interpreting the relationship between these two characters lies for readers.
So we indeed have two aspects to answer, that are difficult to separate in the debate:
- Why do many readers choose to explore the "San/San" relationship under romantic aspects, why exactly this relationship in the books, why not another one, what does assuming a special relationship trigger in us modern readers? What do these two concerned characters appeal to in us today people?
- as immanent debate, what exactly is the nature of that relationship and how is it a different quality from other
relationship in the books?
So what does Sandor/Sansa have to offer to us modern readers? I guess it is a quasi natural or deeply acquired instinct to look out for "romantic" aspects when following a story. We want complete arcs, we want closure and we want to know "how it ends", the stable, peaceful and satisfying government, the longterm relationship. We are looking for it and consciously perceive it when Martin refuses to fulfil the expectations we have learned as readers. Even the most gritty thiller is expected to have something to satisfy the readers in order to sell and the unhappy ending has to be balanced by extremely good writing, at least for me.
So looking for a love story is natural, but why this one? Why do especially many women find the relationship between a multiple murderer and an inexperienced naive thirteen year old so very intriguing? Why do they not choose an older female character, a more worldwise and less defenseless heroine, with a sex life already existing? Why not the very adult beauty and beast story between Jaime (inner beast) and Brienne (inner beauty) ? Because a heroine inevitably has to be beautiful to make her worth of readers' consideration as object of fandom shipping and identification?
Sansa as character is presented as a white sheet at the beginning of the story where the author with all cruelty scratches the scars of events in, there is not very much there at the beginning of the books, she is formed by the progress of the books. This is imo the reason why her POV'S are soo very well written and so much more interesting than the character herself. At the same time readers can choose what to project onto this character, Sansa can be all of us, in her naiveté at the beginning we can see Martinworld through her eyes, while she is not understanding a lot, we gaet the informations indirectly presented and are invited to draw our own conclusions. It's the Forrest Gump writing trick. And we as readers grow with Sansa, the books are not only her coming of age, they are ours as readers as well. We are subtly introduced to the cruelty of Martinworld, culminating in Ned's beheading, being further disillusioned, learning about scheming......Sansa has been especially created by Martin as identification object, as easy story access. We all can be Sansa.
I, as a woman, have though some problems with her character, concerning the emphasis on her helplessness and passivety, she is simply very different from me at her age. And the contrast between the two sisters is wonderfully done. That's maybe only me but a more seriously problematic topic is the fixture on choosing a so very explicitely virginal heroine for identification ( not that I would want a thirteen year old having sex, I HATE child porn and the idea of abusing kids), here it is maybe again the blank sheet readers find easy access to for their projections - and it may be Martin clinging to the topic good (nonsexual) girl - bad ( experienced ) girl. So readers and the author find a common interest in having the cliché virginal heroine with more than believable plot armour against sexual abuse and not the, hopefully older, heroine who is master of her sex life.
Now to Sandor: I must emphazise here that I believe Sandor to be a truly interesting character, his childhod scars, his issues with violence, all that........ Everything has been discussed in these forums.
Beauty and the beast? What is the promise in a topic like that for (female) readers? It is maybe the wish for a protecting hero who is exclusively a good guy to you while showing his aggressive side to the rest of the world, thus contrasting with the chivalrous hero who is supposed to be just and helpful with everyone, so per definitionem he is not yours alone, he has obligations to the whole world. The beast hero is exclusively your own, you don't have to share him since nobody sees his other side which is a secret between you and him. You have private access to the positive parts of his soul, you as woman are the chosen one to reform, to redeem him and this gives you power over a dangerous beast. Importance to the powerless, Sansa's personal Hound, better than a direwolf.
You see the other side of this: in RL the women who feel the resposibitlity the help the villain, to tame their personal beast, to take the task to reform an alcoholic or wife beater are those who may end up as the most likely victims of domestic violence since they are the ones who are never able to seek help for themselves, to go to the police and to turn their back on the abuser.
Though being so very hateful against child abusers I have to defend pedosexuals here to a very certain degree.
First I think that a sexual attraction of Sandor to Sansa is not something I hate in these books. On the contrary, this existing desire adds to that complex literary character and is an interesting invention by Martin. Though I think Sansor's sexual longing for Sansa is not mainly determined by her being physically young but by being naive and inexperienced. He does not want to break her body because she is nearly a child, he wants to break open her soul, he wants to have part of something he lost when he was six. Destroying it in the act while being verbally and physically abusive and only just refraining from being a rapist at the last moment.
There again the aspect of exclusivity: the purity Sandor has lost forever, he thinks to find that in Sansa, and only in Sansa of all people around. And while knowing he can never get it and purity is doomed he tries to destroy it in Sansa, against better knowing, because this is exactly what he desires in her. If he ever got what he desires he would destroy it in the action. I think Sandor knows this.
And so I think Sandor is not a typical pedosexual though craving for childlike innocence. And apart from that, and though having my own ugly experiences when I was eleven ( I WON, the abuser lost because I spoke up publicly and defended myself with the help of my mother. I was very proud of it back then, it made me stronger having won, not ashamed at all), a pedosexual is a child abusing perpetrator only as soon as he shows any - internet, photgrapy, peeping tom, threads or direct violence or whatever - activity in this direction. As long as a pedosexually oreinted person is aware of the danger he or she presents and seeks help he or she deserves not society's moral condemnation but support. So discussing here about "creepy desires" should be carefully separated from assailing actions against young people - and from giving the debate a prudery turn of sex as sin. ( though Sandor had a knife at Sansa's throat and threatened to rape her and later to rip her heart out)
Sandor's desire for Sansa thus personifies the best but as well the most dangerous and destructive of his personality and him somehow knowing this makes the tragic story hugely intriguing. There is nothing to romanticize in it, certainly not material for the erotic identification of fangirls, there is an enormousd darma and inner force in it, potential to doom both characters, to make them go down together, the erotic aspect is no more than a sidenote here.
Thinking about future plot developments, immanently in the books:
In post # 49 it was metioned:
Sandor sees (in Sansa) what she is, Petyr what she may become
Meaning Petyr and Sandor see Sansa under a totally different perspective and I would definitely not see that Sandor's viewpoint has to be more becoming for Sansa's safety than Baelish's. Let's not forget that once little Petyr was the same Innocent and brave boy Sandor has been, only to be brutally disillusioned as well, but drawing very different conclusions for his further life. Sansa may become the heroine actively living her sexuality and using it to get what she wants, may it serve goals perceived as morally good or morally evil by us readers, depending on our likes and our favorite characters. Using her beauty as a weapon would be a fine change for Sansa since until now it only has been her weakness, making her the object of unwanted sexual attention or serving as extra nice parcel wrapping for political claims. I am not willing to morally condemn a female character for making use of her sexuality as weapon, seeing this as somewhat more immoral as slitting throats and chopping off heads. When the man has a knife and the woman has her c..t, use it, I am with you Cersei! ( see, I finally managed to mention my third favorite character in this post!
)Though being the player many readers want Sansa to be may finally condemn her in Sandor's eyes, making him destroy her.
Or he may be her personal savior, sacrificing himself for her, Martin knows. Most posters agree here that a "happy ending" as in happily ever after between those two characters is very unlikely.