For what reasons does Miss Maudie sympathize with Arthur (Boo) Radley?

Miss Maudie knew Boo as a young man and liked him. She felt his family had tucked him away from the rest of the world, and that they were over zealous in keeping  him away. When Scout asks if Boo's crazy, Miss Maudie replied, "If he’s not he should be by now. The things that happen to people we never really know. What happens in houses behind closed doors, what secrets-"