At the beginning of Chapter 7 in To Kill a Mockingbird, Jem is feeling moody and withdrawn primarily because he is grappling with the changes that come with growing up. This chapter takes place after the events of the previous summer, when Jem and Scout had significant experiences related to Boo Radley and their perceptions of the adult world.
Additionally, Jem's mood is influenced by his developing awareness of the complexities of human behavior and social dynamics in Maycomb. He is beginning to understand more about morality, empathy, and the realities of prejudice, which can create an emotional distance from his childhood innocence. His moodiness is also a reflection of his evolving relationship with Scout as they both negotiate their roles in a changing world.
Overall, Jem's demeanor at this point signals the theme of childhood versus adulthood and the loss of innocence that runs throughout the novel.