"Mending Wall," by Robert Frost, is a narrative about two neighbors who meet every spring to repair the stone wall that divides their properties. The primary symbol of the poem is, of course, the wall, and the theme centers around the wall and how each neighbor perceives both it and his neighbor.
Each spring, the narrator contacts his older neighbor and they walk the wall, replacing the rocks that have fallen. It seems a harmless enough pursuit:
I let my neighbor know beyond the hill;
And on a day we meet to walk the line
And set the wall between us once again.
We keep the wall between us as we go.
To each the boulders that have fallen to each.
And some are loaves and some so nearly balls
We have to use a spell to make them balance:
'Stay where you are until our backs are turned!'
We wear our fingers rough with handling them.
The narrator points out that the pursuit is not necessary or even worthwhile, for the fence keeps nothing particular either in or out:
There where it is we do not need the wall:
He is all pine and I am apple orchard.
My apple trees will never get across
And eat the cones under his pines, I tell him.
The narrator seems to think his neighbor is less enlightened than he is, for he calls him "an old-stone savage" and suggests the man lives in a kind of "darkness," as if he is a relic who is not as enlightened as the narrator. The neighbor is rather laconic, saying only the same thing his father used to tell him, "'Good fences make good neighbors.'" The narrator claims this is a ridiculous thing to say.
The primary theme of this poem concerns barriers. The narrator speaks as if he is superior to this unenlightened man and neither needs nor wants barriers; however, it is clear that the narrator clings just as unreasonably to the wall's existence as his close-minded neighbor.
The narrator is the one who instigates the mending every year. Though he is wryly dismissive of the need for a wall, he is certainly an active participant--even an instigator--in keeping it in good repair. He claims that he would never do something mindlessly like his neighbor.
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.
Something there is that doesn't love a wall,
That wants it down.
It is true that the narrator does not repeat a worn-out phrase from generations past (like his neighbor), but it is also true that he is as deeply concerned about keeping the wall in good repair as his neighbor. The narrator begins his narrative with this line, "something there is that doesn't love a wall," and yet he clearly loves the wall and what it represents.
For a more extensive discussion of this ironic theme, see the eNotes link, below.