Back To Black
At the beginning of Order of the Phoenix, we find Sirius hiding out at Number 12, Grimmauld Place. Not only is it a dark, dirty, and cramped little house, but it's also Sirius's childhood home.
And staying there brings back tons of bad memories:
SIRIUS BLACK: This is the Black family tree. My deranged cousin. I hated the lot of them. My parents with their pure-blood mania. My mother did that after I ran away. Charming woman. I was sixteen.
Yeah, Sirius has some issues with his kin. He totally broken from his family and joined up with the Order of the Phoenix to fight Voldemort. His relatives were not amused to say the least.
So Sirius found a new family—his friends in Gryffindor. And, for Sirius, James Potter was like a brother from another mother. You know, a mother who wasn't obsessed with pure-blood wizards. When James dies, Sirius transfers all that love to Harry. He's the kid's godfather and he really wants to be there for him.
Except for the fact that he's also an escaped convict wanted for murder. That makes it tough to act like surrogate dad of the year.
Still, Sirius really tries to have Harry's back. He's the one who wants to give Harry more info about Voldemort's plan while the other members of the Order of the Phoenix veto it. He follows Harry to Platform 9 ¾ in as Padfoot (his Animagus dog form). He pays Harry a visit through the fire in the Gryffindor common room. And, in the end, Sirius comes to fight by Harry's side in the Department of Mysteries.
But throughout all this it's not really clear that Sirius sees Harry as his godson rather than a replacement for his best friend, James. Even Mrs. Weasley reminds Sirius that Harry isn't James. And then, of course, there's that weird moment during the Battle of the Department of Mysteries when Harry disarms Lucius Malfoy and Sirius shouts,
SIRIUS: Nice one, James.
At one point Snape tells Harry that Sirius is a "sentimental chil[d] forever whining about how bitterly unfair [his life] has been." That's harsh, but we've got to admit, there is a sliver of truth to it. Sirius is seriously stuck in the past.
Think about it. He lost twelve years of his life wasting away in Azkaban for crimes he didn't commit. Now that he's out, he's trapped in a place he hates—his childhood home. He's constantly reminded of the friends he's lost and the war that's about to come. Wouldn't you rather think of the glory days you spent kicking around Hogwarts with your best mate, James Potter, too?
Sirius' life has been bitterly unfair. That's why it's such a bummer when he gets killed right in front of Harry. Now they'll never be the family Sirius always wanted. He'll never get to live out those happy days looking after his best friend's son. And Harry will never have the father figure he's wanted all these years.
What? We're not crying. You're crying.