Why is Jem moody at the beginning of chapter 7? Is Scout able to understand by ''climbing into Jem's skin''? What does the rest of the chapter reveal about the source of Jem's moodiness?
At the beginning of Chapter 7, Scout tells us that "Jem stayed moody and silent for a week." That's a hint for us to go backwards a little bit to find out what already happened to make Jem feel moody.
We saw back in Chapter 5 that Atticus told the kids to leave Boo alone. But then in Chapter 6, Jem tries to sneak into Boo's house. (He has to make a quick getaway, and he leaves his pants behind when he gets caught on the fence.) He feels guilty about it, especially because his dad told him not to do it--and his dad has never had to punish Jem physically (by "whipping" him or giving him a "licking," as the kids call it). "We shouldn’a done that tonight, Scout," he says.
Now it makes sense why Jem is "moody and silent:" he feels sad and guilty for disobeying Atticus.
Scout tries to see things from Jem's point of view ("climbing inside his skin") by imagining how she would feel if she had been the one trying to sneak into Boo's house: "if I had gone alone to the Radley Place at two in the morning, my funeral would have been held the next afternoon," she says. But she mostly just leaves Jem alone; she doesn't understand him any better by trying to step into his shoes.
Later in the chapter, we find out that when Jem went back to get his pants, they weren't exactly where he left them. They had been folded neatly--and the hole in them had been sewn up! That reveals that Boo actually helped Jem: didn't want to creep him out, didn't want to get him in trouble, and actually wanted to help him keep the secret by fixing the pants. What a change--Jem had thought that Boo was a creepy bad guy, but then he learned otherwise because of the pants. Because Jem realizes that he had misjudged Boo and treated him badly when Boo had been kind to Jem, now we understand even more why Jem is so moody.