Why was the wall put in place?
We are not told why the wall was put between these neighbors' properties, but we can make some inferences from this poem and speculate a bit. We know that the narrator did not put up the wall, which we can infer from his disapproving attitude about it. There is some evidence that suggests the neighbor's father may have put up the wall because the neighbor is fond of quoting his father as saying "Good fences make good neighbors" (line 27). Because the wall is clearly old, it is also possible that at some point in the distant past, there was a need to pen in cows or sheep on one property or the other, or that there was a feud between landowners that made the wall a good idea. And also, in a terrain in which no natural feature, for example, a stream, marks a boundary well, people do like to know where the boundary of their land is, to know what they are responsible or liable for. It is this sense of good fences being good neighbors that the neighbor is probably referring to. The neighbor is not completely wrong, in spite of the narrator's rant on the topic. One suspects that the neighbor is unlikable in other ways, as well, and the narrator resents spending even one day a year with him in wall-mending.