You've got some great posts here. The title tells all--"Mending Wall"--it doesn't just apply to the mending/repairing of the wall itself. It also refers to the meeting in the middle of two people with differing opinions and viewpoints to "mend" or at least come to an agreement to disagree point in their relationship. It is a peaceful ceremony, they each meet on their respective sides of the wall, not talking much, but coming to an understanding that each has his own ideas of the purpose of the wall. One uses the wall to keep things in, the other to keep things out, but they work together each season to keep the wall in good shape. Thus, "good walls make good neighbors".
I agree with the irony that kiwi explores in #6. I don't think it is clear what Frost himself thinks about metaphorical walls, and clearly the two characters in the poem, the speaker and his neighbour, represent two sides of the argument. The way in which the neighbour keeps on repeating "Good walls make good neighbours" seems to infer that he is rather unthinking about his adherence to the need for boundaries, whereas the speaker seems to question such beliefs.
There is an irony, I think, in the two men having the opposing views on the nature and purpose of the wall. Frost's poem shows that despite the neighbours differing opinions on the function of wall, both use the task of 'mending' it to cement their relationship.
I don't think the speaker is talking about walls in a practical sense. He is not talking (I don't think) about cows or trees. I think the speaker is talking about the need for people to maintain a separation between themselves and society. I think he is saying that walls between an individual and society are important. It is important for a person to maintain some psychic and emotional separation between himself and society. By doing so, people can maintain a sense of self and yet still be able to engage with society on their own terms. In this way, "walls" (in a metaphorical sense) are useful and desirable.
I agree with the excellent analysis of the previous poster. While the poem is entitled "Mending Walls" and the only real action of the poem is two men working on either side of a wall, mending the damage of the winter, it seems to me the poem is not really about a wall. The wall is the thing, as suggested above, which is an indicator of two ways of looking at life. The older neighbor takes his walls quite seriously, clinging to the tradition of the wall despite the fact that it is not keeping anything out or in. The younger neighbor is respectful of the tradition, obviously, or he would not keep doing this annual, rather thankless task. It is two perspectives on tradition.
I believe that Frost is saying there is a place for a fence. There are cows to keep in, but I believe that Frost or the speaker of the poem is not necessarily in favor of the fence between his house and his neighbor's. Of course, the neighbor would not have it any other way, believing that "good fences make good neighbors."
The speaker seems to think his neighbor is from some ancient time period. The neighbor lives and swears by the fence. It has been a tradition in his family. The fence is necessary to keep a peaceful relationship with your neighbor.
Who doesn't like a wall? I believe the speaker believes fences are only for practical use. He even mentions that his trees are not going to get into his neighbor's yard. Also, he says one should really consider what the purpose of the fence is serving. He says one should think twice before building a wall:
Before I built a wall I'd ask to know
What I was walling in or walling out,
And to whom I was like to give offence.
Something there is that doesn't love a wall,
That wants it down.' I could say 'Elves' to him,
The speaker is being facetious, kidding with his neighbor about the elves. He thinks his neighbor is old-fashioned in his desire to have a wall between them, but the speaker humors him and tends to mending the wall each spring:
I see him there
Bringing a stone grasped firmly by the top
In each hand, like an old-stone savage armed.
He moves in darkness as it seems to me~
Not of woods only and the shade of trees.
He will not go behind his father's saying,
And he likes having thought of it so well
He says again, 'Good fences make good neighbors.'