He knew what he had said that night that broke her.
"I don't love you the way I used to."
It was the heat of the moment, it was the cut that was the deepest, it was the one regret he would never be able to get over. Of course he didn't love her the way he used to; he loved her more now, he loved her better.
He loved the way that she would suddenly decide to make cookies, but that she never ate a single one. He loved the way she was able to quote The Notebook, and the fact that she still cried at the end. He loved the way she would come home after work and immediately run into his arms, like everything she did was for him, was because of him.
He loved her so much he could barely stand it.