The Red Wedding never happens, Dany takes the throne
He smiles at her, this handsome little smile he always had smiled when they were children. She loved that smile, once. It told her that everything was okay, that her brother was here and that he would protect her from the evil in the world.
Now, however, it just reminds her that he had failed at exactly that task.
Somedays, when she sees this smile, she grows sad, but more often she grows angry. And even more often, she hates him when he smiles at her like that. As if she is still the little girl that had gone to King's Landing. As if she hadn't seen her father being beheaded, as if she hadn't had to suffer Joffrey and his vile Kingsguard, as if she hadn't survived riots and battles. As if he had saved her.
That's what is hardest to forgive. That he didn't save her. That he didn't trade Jamie Lannister for her, that he left her in King's Landing while he took castles and won battles. It had been Daenerys Targaryen who had sent her home, at last, as a peace offering, after Sansa had convinced her that she could not beat Robb in open field. Robb still doesn't know that she had been the one to tell the Queen that, that she hadn't waited quietly. She had been so sick of waiting, so sick of sitting still and hoping for a white knight to save her. She had lost all her faith in men there in the capital, but Robb never noticed.
She hates Robb when he smiles at her like this, because it reminds her that he doesn't know anything about who she is now.