The speaker seems to feel that, because the gaps keep forming, there is some element in the world "that doesn't love a wall." Nature "sends the frozen-ground-swell under it" so that the rocks on top spill off and land on the ground. Hunters and their dogs, also, must make some gaps as well, as the narrator says he frequently has to make repairs when they come through. However, he says,
The gaps, I mean,
No one has seen them made or heard them made,
But at spring mending-time we find them there.
So, each spring, the narrator and his neighbor pick a day to walk the line of the wall and "set the wall between" them again, repairing these gaps that appear throughout the year. They "wear [their] fingers rough with handling" the stones, some of them quite large, and this is frustrating, it seems, for the narrator, because "we do not need the wall." He reiterates to his neighbor, "Something there is that doesn't love a wall, / That wants it down." Nevertheless, his neighbor insists, time and time again, that "Good fences make good neighbors," and the narrator carries on, really, only because that is what the two have always done. He helps to repair the gaps because it is routine and keeps the peace.