Lancelot and Elaine is one of the more illuminating stories of Lancelot's life, in my opinion.
Lancelot could have loved her. I think he might even have wanted to, despite the sordidness of their early times together. Afterall, he did TRY to make a life with her. In some versions, he even marries her (which, to me, makes sense from the persepective of Elaine's character: good christian lady, etc).
But Lancelot loved the wanting more than the having: he spent half his life searching for the holy grail, even when he knew he could never find it because of his desire for Guinevere. He spent the rest of his life wanting Guinevere, and when he had her he repeatedly does things that will drive her away from him (I know, that's all a matter of perspective: Whyte and Mallory always portrayed their fights as misunderstandings on Guinevere's part, but really: she was supposed to be a wise and good queen, she couldn't have been as dense and spiteful as she's portrayed there)... anyway. Lancelot believed he could have been happy with Elaine, but once he had the happy home, the castle, the knights and servants and maidens, and the lovely little son (the future finder of the grail), he lost spirit. He lost the one thing he had always had throughout his life in every version of the story I've ever read: he lost his desire.
It was like he'd achieved everything, then, and there was nothing left to live for, so he clung to his longing for Guinevere, weather he still loved her or not, because it was the only thing that kept his life from being complete: i.e., over with. He stares out to sea, looking for an ideal that he can reach for. Maybe Guinevere. Maybe his grail. Maybe purity of heart and freedom from sin. Maybe something else, but definately something that will always be beyond his grasp.
Lancelot is the seeker of ideals, and he never matured enough (except perhaps in the aftermath of Camlann) to realize that ideals cannot be found in this world.
Elaine, on the other hand, always knew that Lancelot was in love with Guin. Right from the begining, when she allegedly tricked him into bed with her, the lie was that she was Guinevere (seriously, what sort of man can't tell the supposed love of his life apart from a near stranger?). But he was a great knight, one who had the skill, if not the heart, to find her holy grail, and then he had a child with her, and good little christian that she was, Elaine could not accept the fact that her first love might not be her only love. She clung to him, because anything else would have humiliated her before her beliefs. Before her god. She watches Lancelot through the curtains of her window, trying to pretend that he could still love her, but knowing in her heart that even if he did, if he ever had, he would never be content with her. He would never be content no matter what.
Elaine is the stubborn heart. She was content with the simplicity of her life. With faith in God, and the laughter of her son, but she fell in love with the symbolism of Lancelot, and that was her downfall.
I guess my point is that both these figures are seeking happiness in abstract ideas, and not in the reality of each other. Their misery is caused when they realize neither of them is willing (or able) to change to suit the other's beliefs about what love was, what a good life was, and, most importantly, what the other person ought to be.
Elaine wanted to marry the perfect knight, not realizing he didn't exist (well, until her son grew up, that is). Lancelot sought the unattainable, and was disapointed to find Elaine all TOO attainable.
We can't spend all of our time longing for something we cannot have, especially at the expence of seeng what it is we DO have. At the same time, we shouldn't cling to what we have just because it should always be ours, especially when whatever it is is only making us miserable... In this sense, Lancelot and Elaine represent two ends of an extreme, and it is important to find the balance.