The main theme of the poem is community. The narrator and the neighbor don't appear to have much in common and do not socialize with one another. As one first reads the poem, the wall seems an extension of the barriers between the two, something that separates them. The neighbor often repeats the saying: "‘Good fences make good neighbors." This suggests that the point of the wall is keeping neighbors separated from one another.

The poet points out though that:

Something there is that doesn’t love a wall ...

The wall tends to partially collapse over the winter and needs extensive repair each spring. Nature, the poet suggests, is determined to bring the men together and destroy the walls they build to isolate themselves. In a sense, nature succeeds; the men, who normally lead separate lives, come together every spring to rebuild the wall. The speaker cannot shake the feeling that the neighbor offers his traditional wisdom without examining it—why should humans build walls between each other? What are the neighbors walling out? What are they walling in? In short, the speaker examines the traditional walls that define communities and alienate even as they seek to protect.

The central theme is whether the wall is good or bad for the relationship between the two neighbors. In a larger context, the theme is about the effect of emotional and physical barriers. The speaker, initially, seems to think that the wall is inherently a detriment, unnatural, something that separates and therefore is a barrier to an open dialogue/relationship. 

However, he does still see and converse with his neighbor and the wall does provide a sense of privacy which is not inherently bad. Also, there is the play on "mending" as both a verb and an adjective. As an adjective, the wall 'mends' their relationship by keeping them in communication albeit physically separated by the wall. As a verb, the act or ritual of the two neighbors getting together to "mend" the wall is an event that brings the two together. 

And even though the speaker finds the wall unnatural, it is he who lets his neighbor know it is time to mend the wall. So, it is ambiguous as to whether he really doesn't want the wall there. His neighbor may be thinking the same thing. Do we need this wall? Does this ritual of gathering to mend the wall serve as our only means of communication? And if so, it is ironic that the ritual to mend this physical barrier is also a ritual of connection.