"Because I don't have a choice!" Remus snapped back. There was a growl in his voice, and he invaded Sirius's space. He was taller even if he wasn't as broad, and he took full advantage of the face that he could press in and look down on Sirius, make him feel smaller. It was an aggressive instinct, and at another time of the month he would have been more reasonable, but the wolf was responding to Sirius's anger and Remus was reacting in immediate defense.
Remus Lupin was never good at standing up for himself unless it was downright aggressive. There was something broken in him, too, after the war, after losing everyone. Even with them here, it didn't fix him. If anything it made him more protective, more afraid to lose them, more hostile when it came to defending how things used to be. He'd mourned Peter for a year, thinking of him as innocent, and that crept into the way he looked at things now.