The narrator does not like walls, which is clear from the first line:
Something there is that doesn't love a wall (1)
He goes on to give examples of what nature wants to do to the wall, to spill the stones and make large gaps in the wall, so we can infer that walls are unnatural as far as the narrator is concerned, actually against nature.
He goes on to point out the foolishness of the wall, to no purpose, since there are no cows to keep on one side of it and since his apple trees are not going to try to interfere with his neighbor's pine trees, by straying over and eating the pine cones.
When he points this out to his neighbor, his neighbor responds that
Good fences make good neighbors (27)
He tries to point out that he would want to know the purpose of the wall before building one, what it was meant to keep out and whom it might offend in its building. But no matter what he says, his neighbor takes refuge in the same line.
The title is meant to be a somewhat ironic play on words, since in mending the wall with his neighbor, the narrator is contributing to an endeavor that is not mending anything, but that is more likely to promote bad feelings and offend nature itself.