I headcanon that Dazai starts to wet the bed semi-regularly after Mersault, but he gets defensive when Chuuya calls him a bedwetter. He brings up the few times it happened to Chuuya in the past and snaps that by that logic, he’s a bedwetter too. This makes Chuuya realize how Dazai feels a little powerless in the face of something he can’t seem to get under control, and instead of taking the bait and arguing back, he softens and resolves to help him figure it out.