This is a great question because it asks us to look at characters in novels who are on the fringe of the action, the outsiders, and ask what they can teach us.
Boo is a recluse, unable to interact with Maycomb citizens on a daily basis. How might that disability (and also perhaps a choice) to retreat from society have kept him innocent and preserved from the lack of compassion and racism of Maycomb? Find some instances of racist, inconsiderate, and other inhumane actions that citizens commit in Maycomb, from Mrs. Dubose, to Cecil (who Scout attacks), to Mr. Ewell, to any other "average" citizens who are viewed as mainstream, upstanding, and normal. Are they more likely to commit these acts because they are caught up in caring so much about what others think of them? Is there evidence that their actions are caused by pressure from others? (Where does Cecil get the taunts to torture Scout with? Where does Mrs. Dubose come up with her awful commentary on Atticus? Clearly, someone has told these people something!)
Think about it: if you are out in society, interacting with others on a daily basis, these things tend to matter:
- your economic status (how much do you make and what are you able to buy)
- your social status (who are your friends and what is your reputation in others' eyes).
If you are constantly caring about these two aspects, it's hard not to let your actions be influenced by others. Boo is immune to this pressure; he is already mocked, pitied, even despised. By his retreat, he's gained an interesting freedom to act as he chooses. He can be freer than perhaps anyone to act with kindness.