In Mending Wall the speaker, an apple farmer and his neighbor "set the wall between us once again" after it has inevitably been broken down by natural events after the " frozen-ground-swell" or perhaps unknown culprits, such as hunters although "No one has seen them made..."
The neighbor is happy to repair the wall although the speaker does not understand his reasons and "Good fences make good neighbours" does not seem a fitting enough reason to the apple farmer. It is irritating to him as he is unable to "put a notion in his head" as to whether they even need the wall. They have trees - no cows!
The speaker feels the animosity of his neighbor, although there are no harsh words spoken and no evidence that it exists; the neighbor's very distant, almost dismissive nature irks the speaker and he describes his uncommunicative neighbor "like an old-stone savage armed..." because he feels the neighbor is much like a harsh, unthinking stone-age brute who would build a wall to protect him from other savages and not hesitate to use his stone wall as a means of "armed" combat.
The speaker is thus offended at the inference - the speaker's own interpretation- of the neighbor's feelings towards him.