I feel that the poet wants to share a special bond with the neighbor. In a way that they always mend the wall in spring. I almost feel that the poet wants to share mending the wall with the neighbor.
Mending Wall also makes a comparison to reality, in a way that people in their everyday lives hold up a personal wall toward others. It seems to me that the poet is trying to send out the message that no body should hold walls up to block others out.
Arguing another viewpoint of the poem. The wall may simply symbolize a form of security.
I do feel though that throughout the poem, the poet seemed to be intimidated by the neighbor. In a way that he never wanted to confront him up front about the wall. The poet would give clues and ask questions, but never specifically told the neighbor anything about the wall.