When Harry had been born, Sirius wouldn't have blamed either James or Lily for saying all bets were now off. For Sirius and Remus (and so it had seemed, Peter) Harry's arrival heralded the perfect example of what they were fighting for. Sirius had nothing to lose but this family of his - not the ones who'd given him his name - but the ones he'd chosen. James need never have left the house at Godric's Hollow, and both of them could have distanced themselves from the Order. But neither of them had faltered in their duty, despite Harry...perhaps even because of him.
If Sirius hadn't been born in this house, he might have thought that parenthood brought with it selflessness. As it was, he could only wonder at what good people is friends were.
"But why do we have this second chance?" Sirius wondered aloud, "people aren't supposed to know what happens to the world when they die. They're supposed to go somewhere....nice!" At least that was what Sirius had imagined. Or perhaps there'd be nothing, and it'd just be like falling asleep "and what'll happen when we die this time?" It didn't make any sense. It didn't seem like a reward, and it didn't seem like a punishment either.
Still, Lily's calming philosophy on life always relaxed him. He drew her closer, "I knew I might not make it through, it's the fact that I did was the hard part. Poor Moony - living without us all for all those years," Sirius looked down at her "I wonder why he didn't get married before, whilst I was in prison."