Perhaps the greatest irony in the poem "Mending Wall" is that the speaker continues to help rebuild the wall even as he realizes he disagrees with its presence. As the poem progresses, the speaker notes how all sorts of natural forces, like the ground and animals, conspire to take down the wall each winter. However, he and his neighbor gather each spring to put it back together. On this particular rebuilding date, the speaker starts to internally question why the wall exists. He wonders why it is needed if he and his neighbor's trees don't interfere with each other's property. He starts to even feel offended, thinking his neighbor is trying to box him out through this wall. Despite the speaker's probably true fear, he and the neighbor meet and put the wall together, almost ritualistically. This is a social experience, though the neighbor's insistence on keeping the wall suggests that he wants to isolate himself or separate his property from that of the speaker. This, of course, is another instance of irony in the poem, because they join together to keep themselves apart.
When the speaker asks himself why the neighbor doesn't consider what he is "walling out," he implies that the neighbor is shutting down community and communication by requiring the rebuilding of the wall. The neighbor can only answer that "good fences make good neighbors," and the imagery the speaker uses to describe the neighbor at the end of the poem strongly conveys the speaker's attitude that the neighbor's view is backward and pessimistic. At the end of the poem, though, there is no real reason to believe the two won't meet again next spring and rebuild the wall all over again.