Why does the neighbor focus on the importance of fences in “Mending Wall” by Robert Frost?

In Robert Frost's "Mending Wall," the neighbor thinks it is a good idea to have a fence marking off each man's property so they will definitively know where the borders of their properties lie. This way, no one will trespass and each man will respect the other's property, causing no conflicts. For these reasons, the neighbor declares, "Good fences make good neighbors."

At the same time that the speaker is exasperated by the neighbor's insistence...

In Robert Frost's "Mending Wall," the neighbor thinks it is a good idea to have a fence marking off each man's property so they will definitively know where the borders of their properties lie. This way, no one will trespass and each man will respect the other's property, causing no conflicts. For these reasons, the neighbor declares, "Good fences make good neighbors."

At the same time that the speaker is exasperated by the neighbor's insistence that they rebuild the fence, he, too, repeatedly pursues his idea: "Something there is that doesn't love a wall." He questions the need to mend the wall as he observes that his "apple trees will never get across" the fence and "eat the cones under his pines." Then, he pointedly asks his neighbor a question about the stone wall they reconstruct each year: Why does this fence "make good neighbors?" His neighbor, who the speaker now perceives as "like an old-stone savage armed," simply repeats his father's saying, "Good fences make good neighbors." Rebuilding the stone fence is tradition, and the man will not break from this customary act.
 
Interestingly, too, while the two men work together, there is a psychological wall between them.