The speaker here does not give the old adage much value --he surmises that the saying became popular where a good fence would keep livestock out of growing crop fields and thus keep the neighbors from quarreling. But in this particular location, where the neighbors have met to repair the fence, the neighbors only were separating two orchards, and the narrator points out that the adage makes no sense here. In a larger sense, Frost is suggesting that allsuch old adages should be reexamined, and that his early New England upbringing should also be scrutinized in light of modern thinking.
The speaker questions the validity of even having a wall, and sees no real need for it; in fact, he goes further in asserting that "Something there is doesn't love a wall." He sees it as an annual fix-it ritual; he would be content to let the hunters and the winter break it down. However, his neighbor in expressing that "good fences make good neighbors" might be implying that both should carefully delineate their respective fields so there's no conflict between them. He may also be suggesting that there's some sociability working together to repair the winter damage -- in springtime rural New England, neighbors may have gone all winter without seeing each other, and this spring ritual might have been one of their few chances to meet.
Robert Frost's poem "Mending Wall" is a meditation on the barriers, both physical and mental, that divide people. The poem is told from the perspective of an individual who, together with his next-door neighbor, ritually rebuilds the barrier separating their properties. Who or what repeatedly tears down the wall is uncertain. Early in the poem, Frost's narrator notes that hunters habitually tear down the wall. The hunters, however, are only part of the problem. A mysterious presence or force seems to oppose the structure dividing properties. To the narrator's neighbor, however, the wall serves a useful purpose:
My apple trees will never get across
And eat the cones under his pines, I tell him.
He only says, ‘Good fences make good neighbors.'
Among the more interesting passages in "Mending Walls" is the narrator's comments on the nature of the manmade structure. In this passage, Frost's protagonist concedes that the wall's purpose is a little unclear, although his neighbor's position is less ambiguous:
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 offense.
Something there is that doesn’t love a wall,
That wants it down. I could say ‘Elves’ to him,
But it’s not elves exactly, and I’d rather
He said it for himself.
In the poem's final lines, the narrator again quotes his neighbor's proverb that "good fences make good neighbors."
Frost's poem represents an effort at understanding the contradictory notion that physical barriers separating people fosters a more amicable environment. Anyone who has ever owned property and decried the encroachment of neighboring communities or homes, or who has lamented the intrusions of neighboring children chasing baseballs across one's garden has contemplated the advantages of physical barriers separating properties. Countries around the world construct lengthy barriers along their borders for the purpose of keeping out people and ideas that run counter to their preferences. The Berlin Wall was an inhumane effort by the Soviet Union and its East German allies to not only prevent the loss of manpower, but to help keep out Western ideas and influences. As awful as was that wall, some in the West figured it helped minimize tensions between East and West by keeping East German citizens at home and not fleeing to freedom. In other words, the Berlin Wall was considered by some as "making good neighbors."
Frost's narrator seems resigned to the wall separating his property from his neighbors. He does not understand why the wall is necessary, and may even be complicit in its destruction. His neighbor, however, remains convinced that keeping one's property distinct from another's is a sound policy.