Interestingly, the poem begins assertively, but dissolves into ambiguity. The recital of the adage "Good fences make good neighbors" is a ritualistic belief, something that is done simply because it is traditional. This unthinking and unreasonable adherence to tradition is a motif in other works such as Shirley Jackson's "The Lottery." Clearly, there is a security that comes from following tradition regardless of any value or significance, and this is, perhaps, the motivation for the neighbor's recital of this old adage.
And, yet, it is the speaker who comes after hunters and "made repair" and he is the one who notifies his neighbor over the hill to meet him on an appointed day. In fact, it is at times an enjoyable task for the speaker to repair the wall as ironically, although they are separated, he can mischievously banter with his neighbor, putting the notion in his head, "Why do they make good neighbors?" Obviously, then, there is something in the speaker, too, that does love a wall or else he would not repair it on his own, or meet with his neighbor for the traditional "spring mending time."
The neighbor desires property separation. He believes in boundaries. Walls protect. Clearly, the neighbor is old-fashioned. He believes "good fences make good neighbors." The significance of this phrase is that neighbors should desire to be good one to the other, and if good fences help this process, then build a good fence.
While the speaker of the poem would tend to not see the necessity of the wall, he humors his neighbor. He restores the broken down wall. Hunters and animals have broken down the wall. Restoring the wall is a good time to fellowship and spend time with his neighbor.
Clearly, the neighbor is motivated by tradition. His father taught him that "good fences make good neighbors."
He will not go behind his father's saying,
And he likes having thought of it so well
He says again, 'Good fences make good neighbors.'
Truly, the neighbor is motivated by old-fashioned ideas. The speaker considers him old fashioned:
Bringing a stone grasped firmly by the top
In each hand, like an old-stone savage armed.
He moves in darkness as it seems to me~
No doubt, the neighbor may only believe "good fences make good neighbors" because his father taught him that it was so. Some people hold on to traditions for sentimental reasons only. Clearly, the speaker of the poem does not see the necessity of the wall. He even mentions that one should really consider what one is walling out by building a wall:
Before I built a wall I'd ask to know
What I was walling in or walling out,
And to whom I was like to give offence.
The neighbor is set in his thinking. He will not change his mind. Obviously, the speaker has ask him about the importance of building a wall. To this, the neighbor repeats his thoughts:
He says again, "Good fences make good neighbors."
This is just the way the old-fashioned neighbor thinks. He learned it from his father.