Both the speaker and the neighbor agree on the benefit of the wall. They both spend time repairing it. More than that, they repair it together.
The speaker, however, does question why exactly "Good fences make good neighbors." He does this, he says, more out of "mischief" than serious disagreement. He just wants to hear his neighbor's reasoning on the subject. But his neighbor doesn't provide him with his reasoning, he just repeats the saying of his father. It could be said that he is a traditionalist or conservative. Boundaries exist because boundaries have worked in the past.