You don't remember the way he used to smile at you. You think it might look like the way he smiles at her, and because it isn't directed at you, the mere thought of that smile squeezes your heart. But even when you see him smiling at her, it isn't the smile he reserved for you and that makes you feel guilty, because you liked that it wasn't the same smile.
You don't remember the smile he reserved for you, but you know it isn't the same. You don't want him to smile that smile at anyone else, but you don't want that smile to disappear from this world.
You want him to smile again.
You want his smile back.