Sirius Black had been in a lot of things in his life: Azkaban, debt, lust and familial love, just to name a few. But in his whole life, even before he was sent to Azkaban, he had never been in true, romantic love. Sure he'd had a few crushes every now and then, and he really thought he'd loved Marlene McKinnon, but after having to live with his happy memories being sucked out for 12 years, any spark that they might have had had long ago died.
And when he was finally freed from Azkaban he knew it would be a long time before his mind was cured of the hell that it had been put through, even with the help of mind healers if he could afford them.
For once in his life Sirius just wanted to live without fear.
When he'd been at Hogwarts that fear had been his parents. The Blacks were dangerous, and Sirius, being a Black himself, was one of the very few people who knew just how saturated their blood was with magic so dark it had turned many of his ancestors insane.
When he'd finished Hogwarts it had been Voldemort. He'd chosen to fight against the monster alongside his best friends, sure, but if his grandfather's stories of young Tom Riddle were true then his fear of the crazy bastard was completely logical.
When Voldemort had been smited by an infant Harry Potter, it wasn't so much a fear of someone and more of a fear of something. Fear itself. If he hadn't been the one experiencing it, it might have been poetic.
And when he'd finally escaped from hell - because that is what Sirius was convinced Azkaban really was - and finally told his godson the truth, it had been the fear of being caught by the Ministry, who would then sent him back to prison. He'd barely escaped with his remaining sanity, there was no way he'd be given the chance to escape a second time.
But he'd found safety and a weird, unconventional family in his godson, Harry Potter. The boy was so similar to his parents it was easy to feel as if he'd known him for years, but at the same time he was so different. All the troubles he had faced in his short life had shaped him into a man.
Perhaps that is the reason why Sirius didn't realize what he was feeling at first.
It started with the urge to write him more and more often, even if the boy didn't have time to reply to all his letters separately. When Harry arrived at Grimmuald this turned into lingering glances in the boy's direction and a desperate need to be around him at all possible times. But despite all these and several other signals Sirius was still hopelessly oblivious to his own feelings.
It wasn't until he was in the Department of Mysteries, being blown into the veil by his bastard of a cousin that he realized. Sirius didn't just love Harry, he was in love with him.