"Same reason I sit with you in the kitchen, I suppose."
Regulus went still. The party moved around them, loud and indifferent, and he stood very still inside it. "And what reason is that?"
Peter looked at him with that expression—the one from three nights ago, the one Regulus had been reading ever since in the dark—and said, simply: "I like being where you are."
The fire threw long shadows up the papered walls. The portraits shifted in their frames. Somewhere on the other side of the room Sirius laughed, that particular laugh, the one that filled every room it entered, the one that had made Regulus feel his whole life like a footnote in someone else's story. He did not look toward it. He kept his eyes on Peter's face and found, as he had been finding with increasing frequency, that this was not difficult.
"You've never said that," Regulus said.
"I know." Peter turned his cup slowly in his hands. "I wasn't sure it would land well."
"What made you decide it would now."
Peter considered the question with the same unhurried seriousness he brought to everything, the same patience that had first unsettled Regulus and had since become something he looked for without admitting he was looking. "Three nights ago," he said finally. "When you laughed."
Regulus said nothing. He was holding his cup very carefully.
"You laughed and then you looked—" Peter paused. Tried again. "You looked surprised that you'd done it. Like you'd forgotten you were allowed." He looked at Regulus steadily. "I've been thinking about that."
The thing below Regulus's sternum pressed outward. He had been trained from childhood in the management of his own interior weather, in the suppression of anything that might be read from his face, but he was eighteen and the training was good and not infallible and Peter Pettigrew had apparently located all of the specific gaps.
Here's a snippet from the drowned rat fic. It's on its way :)