In the end, the changes that bring forth the end of Toris' love are simple and small. They're things like short responses when he talks to her and a softer glare and quiet acknowledgements when they end up together in an elevator, or maybe a bar. It's her way of feeling pity for him, and herself, because perhaps she's finally realized how similar their situations are. It's her well-intentioned efforts to make him feel better, and to make herself feel better. She secretly hopes that the world will give her her dues; maybe karma exists and maybe Ivan would notice her too, just like she went out of her way to notice Toris. It's a misguided attempt, sure, but she's changing.
Toris changes, too. Those "hello"s of Natalia's were only fragmented dreams for him, and to hear them in real life shatters some unspoken truth. Toris has lived in a dream his whole life, and it's possible that she's blurred the boundaries between reality and midnight wishes.
So he changes, bit by bit, to counter it. He forgets the silvery-blonde hair and all of the memories, extending back to his childhood, and he forgets how lovely her rare smiles were. He sheds the years he spent lying awake at night, and loses himself with all that time. He becomes someone else.
The next time he sees Natalia, it's through objective eyes that see now that her lips are too thin, and her eyes too cold and dull. And she doesn't note it, but Eduard does, and he says, "What's wrong?"
"Nothing. Nothing at all."
And gone with the restless nights are the anxious days he spent as a lost boy in the new world. He carries himself differently. He's still got the tired look, but his weariness is gone. As though he wants to start anew. Is eager to start anew.
"You've changed," says Eduard.
"Yes," Toris replies. "I don't love her anymore." Maybe he never did.