You've been ignoring Zenitsu lately because of an argument yet both of you didn't apologized to each other. He was badly craving for your attention so he tried to make you jealous for your attention. He was being close with a girl but he didn't really want to be with her. He just wanted your attention. You saw him with a girl and Zenitsu saw you, so he went closer to the girl and he saw you glaring at him. He gave you a smirk, and leaned closer to the girl. She soon leaned in, kissing him.