Something there is that doesn't love a wall,
That sends the frozen-ground-swell under it,
And spills the upper boulders in the sun,
And makes gaps even two can pass abreast.
The work of hunters is another thing:
I have come after them and made repair
Where they have left not one stone on a stone,
But they would have the rabbit out of hiding,
To please the yelping dogs. The gaps I mean,
No one has seen them made or heard them made,
But at spring mending-time we find them there.
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.
Compare the Greek legend of  Sisyphus,  condemned to push a boulder to the top of a mountain, only to have it roll back down each time. (See Homer’s  Odyssey,  Book XI.) 
 Every year at “spring mending-time” the two neighbors return to repair the wall that doesn’t need to be there in the first place–on and on, endlessly. One gets the sense that the speaker may be feeling frustrated (walled in?) by the nature of his task. 
 
 Titian,  Sisyphus,  c. 1548
Mending Wall