Jem forms a new theory about why Boo Radley has never left his house in years. What is this?Thankk you :)

In Chapter 23, Jem and Scout had been discussing Bob Ewell's behavior with Atticus.  Bob Ewell spat on Atticus after the trial, and the kids were concerned for their father.  What followed was a general conversation, first with Atticus, then with Aunt Alexandra, on the nature of people.  Aunt Alexandra, always desperate to make her family seem important, tells Scout she can't play with Walter Cunningham because he is "trash."  Scout is furious, and neither of the kids are buying it.  Alexandra's words are fundamentally at odds with everything Atticus has taught them.  Jem and Scout retreat to Jem's room where they continue their conversation on the nature of "folks" and Jem tells his sister:

 "If there's just one kind of folks, why can't they get along with each other?  If they're all alike, why do they go out of their way to despise each other?  . . .I think I'm beginning to understand why Boo Radley's stayed shut up in the house all this time. . . .It's because he wants to stay inside."