Peter never would have considered himself one of Tony's silver linings, so the suggestion he was is enough to send heat into his face. He looks down and away, trying to look less embarrassed than he feels, but just makes it all the more obvious he's embarrassed to begin with.

"What are you unhappy about?" he asks, once he's managed to reign in his embarrassment. Peter can't understand what he'd have to be unhappy about. Tony was smart, he was rich, he was handsome, he had a really, really nice girlfriend/fiancee, and he was Iron Man. What could he possibly be unhappy about?