Why does the neighbor say that “good fences make good neighbours” in "mending wall"?
he doesn’t want cows in his fields.
he is repeating what his father used to say.
he is worried about people being on his land.
he does not like the poem’s speaker.

They neighbor says that phrase because he is repeating what his father used to say.

In "Mending Wall", the phrase "good fences make good neighbors" is used twice. The second time it is used, the phrase is preceded by "[he] will not go behind his father's saying (...)".

"He" refers to the neighbor, "not go behind his father's saying" refers to the fact that the neighbor will strictly follow his father's saying which was "good fences make good neighbors".