There are positive and negative aspects to the wall and this adds to the complicated notion of the relationship between the two neighbors. Every spring, the two neighbors meet in order to repair the wall. On one hand, the fact that they continue to mend and sustain a wall between them suggests that they want to remain separate, that they would not think of sharing the borderland, and that therefore, they are not on friendly terms. On the other hand, they are willing to meet every spring. The narrator says it is "just another kind of outdoor game." The notion of it being a kind of "game" shows that they do not rebuild the wall out of mutual hate. Rather, they seem to treat it like a mutual play, something they might even enjoy.
The narrator asks why fences make good neighbors. The poem provokes thoughts on both sides. The wall separates the two men, but the annual rebuilding brings them together. In effect, the wall itself is what brings the men together each year. It separates and brings them together. The poem is complicated and presents the wall in this paradoxical way. Does it prevent friendship? Does it annually mend the possibility of friendship? Does it simply sustain a cordiality between two men who are simply not suited to be friends?
There is no direct evidence to suggest the two men are friends, but they might be. The speaker's comment about their differing orchards might also be a comment on the two men themselves. "He is all pine and I am apple orchard." This suggests that the men are quite different. So, the wall is just a necessary boundary between two different orchards and two very different men. There are no logical or ethical reasons to force these two men to be friends and/or share the land.
The title "mending" is open to different interpretations as well. Perhaps they were friends and this annual rebuilding is a symbolic gesture of rebuilding that previous friendship. Again, there is no direct evidence for this. It is just a hypothesis. Even if they are not friends, the annual mending of the wall presents the opportunity to be "friendly" even if it is just once a year. In this respect, they are (functionally speaking) friends once a year for the limited time it takes to mend the wall.