Discovery.
Sirius is and has always been a curious lad. He couldn't help it, he had to know, had to know peoples motives, how they feel about him, their secrets, how they think. He needed to know or else it would drive him insane. His mind would fill the blanks with cruel thoughts, it wasn't very nice so he would try to find out the truth.
...
Sirius couldn't take it, he needed to know what Remus was hiding. He had an idea but he couldn't be sure, Lycanthropy. It was just a theory but it made sence, disappearing every full moon, more agitated around the full moon, ate less around the full moon.
Now Sirius didn't always take well to being yelled at, so he yelled back. Everytime around the full moon Remus and Sirius would fight constantly. It's never(almost) gotten physical, but James breaks them up. Sirius likes that about James, always so kind.
Sirius didn't understand why he felt different around James, maybe it was just because he was his best friend. He was curious, he wanted to know what these feelings were. He decided to ask Ms. Mcgonagall about it.
...
In Sirius' first detention he was bored out of his mind. He was finished with his work, he had nothing to do. Then he remembered what he was going to ask Ms. Mcgonagall, "Ms. Mcgonagall?" he pondered hesitantly. "Yes?" she answered, not looking up from her work. "Is it normal to feel different about someone then anyone else?" Sirius asked, he wasn't nervous persay....well maybe a little. Sirius watched as Ms. Mcgonagall paused her work, it made his anxiety spike. She looked up at him curious, "What exactly do you mean, Mr. Black?" she said.
Sirius winced at the use of his last name, not sure if Ms. Mcgonagall picked up on that or not. "I mean...like when your around them or think of them you get this warm feeling but not with anyone else, do you know what i mean?" he said, not sure how to put into words. Ms. Mcgonagall seemed to understand his question more now.
She did pick up on his wince and decided not using his last name was better, "Sirius, it means these feelings could be romantic." she explained. Sirius was dumfounded at the use of his first name and suddenly ashamed at her explanation, "Is it weird to have these feelings for a....boy" he said, staring at his desk.
Ms. Mcgonagall's face softened, "No Sirius, it's not weird. it's completely normal and acceptable." she said. Sirius felt sudden relief but had more questions. The rest of his detention consisted of Sirius asking Ms. Mcgonagall questions, by the end of it he had figured out a lot of things about himself.
He found out he likes boys, not girls. He liked James, no one else. This was a problem. This wouldn't do at home, father would be furious and mother.....it wouldn't be good. He had to hide this, hide this forever. It will be his secret. His deepest, darkest, loneliest secret.