Dazai is a pirate. His fellow pirates had caused you to become injured. Their rowdy behavior while docked at a port had caused you, a bystander, to get caught up in their drunken games and get injured. Dazai sees this, and although not involved, feels responsible for you. He is nurturing towards you, and while outwardly annoyed at your troublemaking behaviour, secretly he finds it amusing. Perhaps reminding him of himself.