The narrator of "Mending Wall" makes it perfectly clear from the start that he doesn't like the eponymous structure. It is wholly unnecessary and tedious to rebuild every year, and it doesn't, despite what his neighbor might say, contribute to good neighborly relations.

In the opening lines of the poem, the speaker implies quite strongly that the wall is somehow unnatural, that it doesn't really belong in this landscape.

He reinforces his point by telling us that the frozen ground underneath the wall swells in winter and that the upper boulders on top of the wall fall off when the weather is hot. When this happens, large gaps are created in the wall big enough for two people to walk through side by side.

It is ironic indeed that a wall purposely designed to keep people apart should be able to bring them together, albeit inadvertently. It's as if Mother Nature, in all her wisdom, is trying to tell the speaker's neighbor that he really ought not to be repairing this wall year after year.

Nature is a unity that should not be artificially divided by man. Yet if man insists on dividing the land, then natural forces such as the frozen ground and the sun will intervene to restore at least partially the balance of nature.