When Sirius met Harry he realized that there were a lot of similarities between James and Harry. Of course, they looked the same. They were both good at quidditch. Harrry had the same sense of chivalry as his father. Yet despite those similarities being quite obvious they were not what struck Sirius the hardest.
What always struck Sirius the hardest was how similar their attitudes were. He even once found himself saying something to Harry that he had told James long ago. "The world is separated into more than good people and death eaters."
James had never quite got that.
Both men (Yes, Harry had become a man- a young man- in his mind now) looked at the world in this naively black and white way. Sirius always supposed that was how they both managed to be so sickeningly noble all the time. In their minds, you were like that or you were bad. They could be no other way,
When James was a first year and he met Sirius and Snape for the first time they were very alike. But the young prat Sirius had been put into Gryffindor for reasons he could not fathom and loathed it. Snape had made Slytherin. Thus in James mind, Sirius was good and Snape was bad.
Snape was bad. He could never be anything but bad, he was Slytherin and he looked bad. Snape could have helped James with his homework, stuck up for him when his hair got hexed blue, or complimented him on his good flying in class, it would have made no difference. Sure, Snape did none of these things, but if he had it would have made no difference, to James, Snivellus was bad.
Sirius in James' mind was good and he could never be anything but good. After all, he was in Gryffindor, and James' general first impression of Sirius was that he was good. Nothing Sirius could do could ever change his mind. Sirus could (and did), glare at him, call him queer (and various worse things), and hex his hair blue.
It was all joking, pranking, fun, to James. After all, they were buds, right?
What could Sirius do but become good with that laughing, smiling face always around, watching his back for him, even if Sirius never returned the favor. Sometimes he would wonder if given the same chance he had from James if Snape could have been good too…
But James thought Snape was bad.
So Snape stayed bad and Sirius found out what it was like to be good. Though he could never manage to be a disgustingly noble as James, it didn't matter. In James' eyes he was his friend and could do no wrong. That unchangeable view of his friends, was James' undoing in the end.
Now Harry had the same outlook.
Sometimes it was unbearable to Sirius how like his father the son was.