Have you ever experienced that someone liked you and you get too excited and act too immature about it when you're supposed to stay calm and just let that guy like you? Immaturity isn't really just an attitude of young ones, it's the lack of understanding and experience. In every action there's always consequences, for Saiji, her immaturity lead her a feeling she never thought she'd feel-heartbreak, and after that heartbreak she'll learn a lesson that everyone can also learn from her.