Even with his dismal outlook on himself, Rosinante knows people in general, and Law in particular, well enough to realize that the anger isn't actually directed at him or anything he's done. Law is angry at a lot of things, has plenty of reason to be. Probably a lot of other feelings come out as anger because it's what he knows best.

Understanding people doesn't mean he knows exactly how to respond, though. Most of the time, with most everyone he meets, his role is to sit quietly, to observe. Sometimes people take this as support, but with Law, one of the few people he's ever really felt close to, he genuinely wants to help him feel better and just doesn't know how. He's good with words up until they need to be honest, and then it's hard to know what to say.

"Law," he tries again, "Thank you. For being someone who would. Who cares." Who loves him as much as he loves in return. Without another word, he shifts his weight so he moves forward onto his knees, which puts him in arms reach of Law, and pulls him into a tight hug.

Which, it turns out, is not a position he can sustain his balance in so really all he does is pull him and Law very abruptly onto the floor.