Sirius couldn't help but cry all the harder when James answered him -- because he remembered all to clearly the conviction he'd spoken with when he'd told young James that they were brothers. He hadn't even thought it was something extraordinary he was saying at the time. It was just fact in his sixteen-year-old self's life.
"I fucked everything up," Sirius said -- and he had. He'd thrown away so much of his life. What should have been the best times spent with Lily and James and Remus, he'd lost all of that. Surrendered it to a lifestyle of hatred and darkness, content to look in the mirror every morning and see some twisted version of Bellatrix looking back at him. And he'd fucked everything up here too, nearly burned every bridge that he'd started to repair, with the exception of the one that he had with James.