"You know better," Ned says. The son he'd raised should have known better, surely, yet there he'd been, on the eve of his wedding to another, twined around the eldest Mormont daughter like a vine about a tree, his hand in her hair and his mouth on hers as if he could merge them into one. Ned's first thought should have been of Robb's promise, of his word given to the Freys with the weight of Winterfell behind it. But at first, Ned had only thought how happy Robb looked, how his touch spoke of such intimacy and familiarity. Then he'd thought of how many hearts would break from that intimacy, no matter what course his son took, and his own heart had felt as heavy as iron in his chest.
"You wouldn't understand," Robb says bitterly. "You'd bed duty and honor like a woman if you could." It's ironic, really, that what keeps Ned from chastising Robb for his words is the fact that he does understand, and quite well. He knows all too well what it means to put aside a woman you love to do what is expected of you. To do what is right. He knows what it is to be haunted by the choices you've made, by the rumors so complex that you have trouble knowing what's true. If he closes his eyes, he can still see her own eyes as if no time has passed, the color of violets in spring, a purple so startling and lovely that by the time a man was done staring at them he'd already fallen in love with no hope for escape. He sees her falling in his dreams sometimes, even still, even as happy as he is with Cat. He sees her body tumbling through the air only to be swallowed by surf and he knows he did not do right by her, no matter what lies are true and which truths are false.
"I understand regret," is all that Ned says. "And one of my greatest regrets was pretending I had a choice." Robb frowns, his brow creasing in confusion, and Ned is reminded once again how young he is, how sheltered his life has been. Maybe Ned has been too indulgent. Maybe his heart has been too soft.
"You are no longer a boy, Robb. I trust you to do what's right." Ned stands, effectively ending the conversation, though he can see the questions in Robb's eyes, can see his dissatisfaction. Ned does trust him to do what's right. It's only that Ned is no longer sure what right truly is, and he's afraid of what the answer might be, if only he were honest with himself.