I believe neighbors should help each other out, but it is not mandatory. In the beginning they didn't realize how good he was to them, but now they are grateful. Scout attempted to give him a thank you letter but was unable to, because Mr. Nathan sealed the knothole before she could leave it there. She may be sad about not being able to repay his kindness, but when people do things genuinely out of kindness, they don't expect anything in return.