How we cite our quotes:
"He's right!" she said; "of course, he's always right; he's a Christian, he's generous! Yes, vile, base creature! And no one understands it except me, and no one ever will; and I can't explain it. They say he's so religious, so high-principled, so upright, so clever; but they don't see what I've seen. They don't know how he has crushed my life for eight years, crushed everything that was living in me – he has not once even thought that I'm a live woman who must have love. They don't know how at every step he's humiliated me, and been just as pleased with himself. Haven't I striven, striven with all my strength, to find something to give meaning to my life? Haven't I struggled to love him, to love my son when I could not love my husband? But the time came when I knew that I couldn't cheat myself any longer, that I was alive, that I was not to blame, that God has made me so that I must love and live. And now what does he do? If he'd killed me, if he'd killed him, I could have borne anything, I could have forgiven anything; but, no, he.... How was it I didn't guess what he would do? He's doing just what's characteristic of his mean character. He'll keep himself in the right, while me, in my ruin, he'll drive still lower to worse ruin yet..." (3.16.6)
Anna argues that Karenin has never once given her love. She has tried to find other means of obtaining love, but ultimately, she must love in order to be fulfilled. And that means a life with Vronsky. Later in the novel, Dolly sympathizes with Anna's situation. She says, "How is she to blame, then? She wants to live. God has put that into our souls" (6.61.17). Though Dolly (and Tolstoy!) doesn't approve of Anna's affair, she can still understand and sympathize with Anna's need for love. Notice how both Anna and Dolly claim humans require love – God made people that way.
She was an honorable woman who had bestowed her love upon him, and he loved her, and therefore she was in his eyes a woman who had a right to the same, or even more, respect than a lawful wife. He would have had his hand chopped off before he would have allowed himself by a word, by a hint, to humiliate her, or even to fall short of the fullest respect a woman could look for. (3.20.3)
In Vronsky's eyes, love legitimizes all. He refuses to disrespect Anna for cheating on her husband.
But he could not at once recall what he had been going to say. These fits of jealousy, which of late had been more and more frequent with her, horrified him, and however much he tried to disguise the fact, made him feel cold to her, although he knew the cause of her jealousy was her love for him. How often he had told himself that her love was happiness; and now she loved him as a woman can love when love has outweighed for her all the good things of life – and he was much further from happiness than when he had followed her from Moscow. Then he had thought himself unhappy, but happiness was before him; now he felt that the best happiness was already left behind. She was utterly unlike what she had been when he first saw her. Both morally and physically she had changed for the worse. She had broadened out all over, and in her face at the time when she was speaking of the actress there was an evil expression of hatred that distorted it. He looked at her as a man looks at a faded flower he has gathered, with difficulty recognizing in it the beauty for which he picked and ruined it. And in spite of this he felt that then, when his love was stronger, he could, if he had greatly wished it, have torn that love out of his heart; but now, when as at that moment it seemed to him he felt no love for her, he knew that what bound him to her could not be broken. (4.3.17)
Vronsky thought he would find happiness with Anna through his love. Even when he learns that his love has nothing to do with happiness, and even when his love for Anna is fading, he feels inextricably bound to her. After all, he believes it's his fault that she has "changed for the worse." In convincing Anna to have an affair with him, he destroyed the woman he loved. After "ruining" her, he feels he can't abandon her. You can find more evidence for this perspective in the scene when Vronsky and Anna first sleep together – he is described as a murder, and she the corpse of his victim.