In To Kill a Mockingbird by Harper Lee, what may have been a reason Jem was often more moody than Scout?

There are three reasons: puberty, perspicacity, and personality. 

During the course of the book, Jem becomes a tween.  He goes from being nine to twelve years old.  As he transitions to early puberty, he becomes more quiet, reflective, and moody:

Jem was twelve.  He was difficult to live with, inconsistent, moody.  His appetite was appalling ... This change in Jem had come about in a matter of weeks.  ... Overnight, it seemed, Jem had acquired an alien set of values and was trying to impose them on me.  (Chapter 12)

A second reason Jem is often moodier than Scout is that, in many of the incidents in the book, he actually realizes what is going on, while Scout (who is much younger) has not caught on.  Some examples:

• In Chapter 3, Jem reaches out to Walter Cunningham, when Scout had no thought but to beat up on Walter for getting her in trouble at school. 


• In Chapters 7 and 8, Jem figures out that Boo Radley has been putting toys in the old hollow tree in order to make friends with the children, and that Boo was the one who hung a blanket around Scout's shoulders as the children stood in front of the Radley house watching the fire. Jem is concerned about protecting Boo Radley, who "ain't never hurt us."  Scout is clueless. 


• During the trial (Chapters 17 - 20), Jem is obviously following the significance of the testimony, while Scout is understanding only a portion of what is going on.


• In the chapters following the trial, Jem is devastated by the guilty verdict and has several conversations with Atticus about "how could they do it."  He also gives Scout his emerging views on the snobbery and racism of Maycomb. 

Finally, Jem has a quieter personality than Scout that includes a tender heart for those who are suffering.