That was it, exactly. Remus had never said anything like that. So besides feeling shitty about Sirius being an ass, he had to deal with himself lowering to that level. And he didn't know how to deal with it, wasn't used to apologizing for something, and since Sirius hadn't taken it well the night before, he didn't want to get shot down again. So he just left instead.
"I just figure I should know ahead of time." He didn't want to be blindsided with irrational anger, though this time it wouldn't be as irrational as it had been last night. Remus wasn't his personal punching bag.