When Scout takes Boo home, she understands many things as she sees the street from a new point of view. Explain some of the things she "sees" now.
After taking Boo home, Scout stood in front of the window "to the left of the brown door." From her new vantage point, Scout was able to see that Boo was probably able to watch the whole street whenever he wanted. As she thought back over the events of her daily life, Scout understood that Boo had formed a protective attachment to her and her brother, Jem, from simply observing them. Boo had no interaction with others, except for a very few close relatives, etc., so he chose to come to know Jem and Scout in the way he was able, as well as to show him affection for them by leaving them gifts and, eventually, by saving their lives. Despite his differences from other people, Boo was kind and lonely and reached out to Jem and Scout in the only way he knew how. His world consisted of that house and his view of the street; Jem, Scout, and Atticus became his family. He loved them because they loved each other.