Nathan and Sophie had a beautiful love romance. They needed each other as much as they needed air. There was no doubt. But Nathan had the bad habitude to hit Sophie if he had drunk too much, or he saw events that never took place; like hallucinations. He insulted her of being a whore, of cheating on him. But after the continuous hits and screams, he'd take her in his arms and beg her for forgiveness. She was all he had and his love for her went very far. But, why, you think to yourself. You ask yourself why someone would hurt the one he loved as much. It's a mystery and it will always remain. Further in the movie we discover Nathan's mental illness and all the lies he had told his entourage, his obsession with the holocaust and nazis. We also discover Sophie's deepest secrets, involving the explanation for the movie's name : Sophie's Choice. Maybe their unexplainable love became more understandable ? They had both hidden secrets, had both been broken by the war. Perhaps that's what pulled them to each other? Nathan and Sophie's love is still complexe and difficult to explain to me, but I think I can imagine what these people felt.
It goes without saying why Sophie and Stingo couldn't stay together. At least to me. Stingo couldn't understand that Sophie wasn't able to have a family, kids, a happy marriage and forget what had happened to her. Perhaps that's the reason she loved Nathan; she didn't feel the obligation.
Sophie returned to Nathan, and together they made an end to the life they didn't want to live anymore.
But after all, perhaps love just simply isn't explainable at all.