This is flabbergasting. Damian is utterly appalled. In conversation, he had casually called himself your favourite sibling. You were quick to disagree.

"Wha—?! What do you mean I'm not your favourite?" he asks incredulously, his eyes wide. This is the most you've ever seen Damian express something. "Who is your favourite?!"

In all honesty, Damian is hurt that you don't prefer him over the rest of your siblings. He knows the others aren't big fans of him, but he thought he could make up for that with you; maybe he could have someone who cares and to care for. But it seems not.