I was just thinking over the question of, why Hurt/Comfort if not S/M (the most obvious explanation being, apparently wrong) and I think I may have a handle on it-- practitioners tell me if thhis expalnation seems to suit. When we see someone hurting, we generally want to comfort them. It's one of the nicer instincts within human nature. Most of the characters on B7 are walking wounded in one way or another: we know primarily of the emotional wounds of Blake and Avon, and can basically guess at Vila's (he's got 'bullied' written all over him). We see Tarrant Cally and Dayna go through some big traumas. So there is an impulse to 'help them'. Problems involved being, you-the-writer can't do it strictly within canon; the stories where they are traumatized are over and done with. You could potentially write an AU where Avon and Cally hold hands and drink tea post CoA-RoD and develop a bond of trust that enables him to talk to her *before* Terminal... but that is AU. Likewise Blake and Avon having heart-to-heart chat post Voices where the barriers fall. Comfort means trust, and there isn't enough trust on screen, so the solution is to create an original plot. This original fanfic then, must reopen wounds or cause fresh ones: hurt/comfort requires a fresh hurt to motivate the characters. There has to be a new vulnerability. Therefore the writers of Hurt/Comfort aren't interested as much in hurting him as they are in creating a motive for the Comfort. The new Hurt is merely a substitute for the Canonical hurt the writer really wants to comfort.
Okay, there is the Beautiful Suffering thing, but as said by afficianadoes, that is based on the dignity and strength he shows on screen. As I have pointed out, this is akin to admiring the courage of Oedipus in pressing to learn a truth he has been told will destroy him in a classic play; the tears of Mary holding Jesus in the statue Pieta; or any art that makes us admire how the human spirit can bear up abainst pain. Admiration of spirit may be translated into desire, but this is a process than sadism.