In "Mending Wall," what is the effect of the wall between the speaker and the neighbor?
As the narrator and his neighbor undertake their annual ritual of wall repair, the narrator shares his belief that the wall acts as a barrier between him and his neighbor. He uses the poem to poke gentle fun at his neighbor, who believes that "Good fences make good neighbors"(line 27).
The narrator says in the very first line that the wall is an unnatural barrier,"Something there is that doesn't love a wall."(line 1). And the ritual just creates a foolish game,
Oh, just another kind of outdoor game,
One on a side. It comes to little more: (lines 21-22)
Since the narrator has apple trees and the neighbor has a pine wood, there is no risk that the apple trees will disturb the pine woods in any way, and thus the wall is pointless. If there were cattle, for example, that strayed onto the other's land or if there were some sort of boundary dispute, the wall might make some sense, but there is no reason at all that the wall matters, but for the neighbor's saying that it does.
Thus while the narrator and the neighbor are engaged in the activity of mending this wall, the wall does not promote any mending between the neighbors. It serves only to divide them and to give the narrator a good reason to vent!