Jem says, "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." Why does he say this? What does he mean?

Jem has begun to realize that the world can be a very cruel place. Empathetic people, "mockingbirds", frequently bare the brunt of the world's ugliness. Jem is sickened by the verdict at the trial. He is so disillusioned with his community that he now understands why a kind soul like Boo Radley would want to keep away from people.