Why does Boo Radley in To Kill a Mockingbird stay inside his house all the time?

The townspeople believe that Boo Radley is a hateful and mean creature of a man and tend to make up Horror stories about him. But as the book progresses, it is discovered that Boo is just a discreet man who enjoys staying inside but has a clear view of right and wrong. He adores Jem and Scout and it takes their lives being endangered for him to fully emerge from the house one night. He kills Mr. Ewell to save them and then carries Jem home and makes sure he is taken care of before heading back home.