Because his mother had blasted him off the tapestry. Regulus wished it was that simple. He wondered how in the world Sirius and his friends' minds could have worked that they could boil every problem, every issue that existed between them, down into a single action. His mother had blasted Sirius off the tapestry. Regulus clenched his jaw, suddenly feeling more alert. What a stupid, little fool he'd been. To think that any companionship could be found here. He had let himself forget. He wouldn't make that mistake again.
Oh, and his denial of Sirius had nothing to do with the fact that Sirius had left. That Sirius had left him. That Sirius had chosen somebody else to be his brothers while ignoring the fact that Regulus was the one that he actually shared blood with. And at fourteen, Regulus had suddenly found himself without a brother - of Sirius' choice, first. It had been Sirius who had made that decision initially, not him, so why did he have to be faulted for it? When it had been him that had to pick up every responsibility that Sirius had left behind and try to make everything all right.
"Don't you feel sorry for me," Regulus said, his voice simmering with anger - anger that he was controlling, holding in. The instant he had finished speaking, he was sliding out of the bed, pushing the covers aside, as he started for the door of the bungalow.