sirius hates that his brother wont come with him, hates that he thinks that their parents love them, hates that no matter how much he explains, no matter how much regulus will even agree that their parents might be unfair to sirius, he'll never believe that they dont mean well for regulus
(but he gets it. becuz sometimes, rarely, but still, his mother would order the house elves to make sirius' favorite food, or would gently brush his hair out of his face, or even compliment him and even though sirius hated her, hated what the black family stood for, fully believed in muggle rights and all that, sometimes a tiny part of him would wonder, if itd be different, if only he'd been better,,, so yes, sirius gets why regulus waits for the moments their mother might look at them, almost warmly, the moments where it looks like she's happy, say that regulus is doing good, that she almost almost loves him, and might love him, if he kept on trying, and that makes sirius hate his mother more. and he wishes, sometimes, that they didnt have any of those almost good moments at all, so itd been easier to pull away)