What is the theme of the poem, "Mending Wall"?

The writer of the Masterplots commentary on the poem notes that the theme of the poem is barriers. To a great extent, that is correct. The action described is that of the speaker and his neighbor doing the annual repair work on the wall between their properties. This wall sets up a barrier to keep their animals in and to keep each other out.

There is a sense as well that the theme might be the breaking down of barriers. The neighbor says twice that "good fences make good neighbors." The speaker says just as often, "Something there is that doesn't love a wall." From this statement and from his description of the repair as being a chore, you get the sense that the "something" that doesn't love the wall is really the speaker, that he would be happy to remove that barrier.