In "Mending Wall," the neighbors repair the wall every spring because "Good fences make good neighbors"--at least, this is the answer the narrator's neighbor gives him when he asks. The narrator doesn't understand it himself; he sees the wall as unnecessary, since "He is all pine and I am apple orchard. / My apple trees will never get across / And eat the cones under his pines" (25-7). The narrator is practical, analytical; he sees no purpose and thus considers this a waste of time. Nevertheless, each spring he notifies his neighbor ("I let my neighbor know beyond the hill" (12)).
There's a clue in the name of the poem. "Mending" is an adjective here, not a verb. That is, erecting the wall mends something between the neighbors. So one of the reasons the neighbors continue to meet and mend the wall is that doing so "mends" and maintains their relationship.
Another reason they do it is because the neighbor's father had long ago passed along to him the adage that "Good fences make good neighbors," a bit of wisdom he accepted and never questioned, and "He will not go behind his father’s saying" (43). That is, he believes there is undeniable truth in it, even if he does not fully understand it himself.