It offended him when Severus called whatever it was he knew to be 'too complex' for him to learn. Remus wasn't one to be prideful, but it was basically common knowledge that he was extraordinarily smart. ...Perhaps just not as naturally when it came to Potions. Nevertheless, even then, he just had to work a little harder to grasp the concept. What could be too complex to learn?
But this wasn't the time to be arrogant. If taking a few insults from Snape meant Sirius could be helped, he'd let the Slytherin mudsling him all day.
"He does want a solution. And if you can help him, whatever your reasons, I would be grateful to you. He.. might be.. too." He shook his head, knowing Sirius was never going to go for this. "But I have to know what it is. After whatever it is he's gone through, I know he won't just trust anyone blindly without knowing what's going to be done to him. At least give me your word that you won't hurt him."