Arthur was a loner who rarely left his house or even allowed himself to be seen, presumably because of the emotional damage that was inflicted upon him by his father when he was a child. Since the two children Jeremy and Jean Louis(Scout)Finch were usually the only ones to come close to his house, they were the only ones he had some sort of connection with. Still, he was too timid to introduce himself, so he simply chose to leave gifts for them as an act of kindness. It is implied that he followed the childrens emotional journey from a distance throughout the course of the tale, thus strengthening his fondness for them.