In "To Kill a Mockingbird", why did Boo want to be friendly, leaving gifts for Jem and Scout? Why was he interested in them?
In his youth Arthur Radley Jr. had known the complicity of friendship. He got into trouble for joy riding, then for (along with his gang of friends) locking up a school official in the outhouse and leaving him there.
Then his father struck a deal with the judge and the school officials that if Boo would not be sent off to technical school (part of a remedial education program), he would keep his son at home "out of sight, out of mind." The other boys who did the program got an education and good jobs later on, but Boo was "condemned" to sit at home and vegetate.
Instead of living life, Boo became a kind of spectator of it. The children he saw playing outside were virtuallly his only contact with the neighbourhood, being under the strict vigilence of his father. Ironically as it may seem, Boo probably appreciated the children's pranks and maybe even witnessed the scissors scene on the Finches' porch. (We learn that he laughed when Scout rolled up to his door in the tire and scrambled to get out.) He considered their interest in him as normal and wanted to respond to it. He might have also wanted to show the children how wrong other people's opinions of him were. Evidently, Boo's contact with the children was more than a simple game, because at the end of the story he risked his life to save both Jem and Scout from Mr Ewell.
Jem and Scout had the kind of childhood Boo probably wished he had had; surely, they had the kind of father he would have liked to have had as well.