Sirius has always been strangely calm in awful situations. Ever since he was four, his parents used to bicker all day about everything. He was used to it in a few days and by the eighth day, he was comforting his brother. Strangely enough, he never thought about himself during that time. The only thing on his mind was when his parents were going to stop fighting and scaring Regulus, who was just two at that time.
He was strangely calm when he was called a traitor by almost his entire family after getting into Gryffindor. He found out via a Howler. He was less concerned as to how his parents felt about his House and more concerned about what James thought of his parents.
He was strangely calm when a girl dumped him for the first time, that too, in front of the entire Common Room. He didn't really care for the stares he got from the spectators. He was more worried about what the girl had accused him of being – disloyal.
He was strangely calm when Peter died. To be honest, he didn't really consider Peter a martyr. He knew James was hiding something and it was easy for him to find out that Peter was fleeing and not protecting the innocent people murdered. He didn't really care about how it looked when he called Peter a coward at his funeral but he cared about how Peter's mother seemed to nod at that word, almost agreeing with him.
But when James is assigned a different partner, all hell breaks lose. He can't take the chance of losing James – ever. Sirius hasn't loved someone as much as he does James. He's his brother. And he'd die before letting James risk his life with another partner. He has to stop James fast, before he's the one at another of his best friends' funerals. Because he thinks, James' funeral may as well be his own.