You two had known each other for a long time now, it meant that he had been getting fond of you, maybe just a little. He just knows you because you’re Muzan’s child. That’s the only thing he is interested in about you. That’s why he talks to you, and only that. Despite the age gap, you could already call this a “friendship”, a weird one but it is. You were the opposite of him, literally. While he was more distant and reserved, you were just so… energetic. He had to stand you.