These specific lines in Frost's "Mending Wall" illustrate the difference in personality between the speaker and the neighbor who wants to keep the wall alive and effective. There is a playful tone on the part of the speaker in his use of a "spell to make [the rocks] balance: 'Stay where you are until our backs are turned!' The neighbor only says "Good fences make good neighbors," which means that he doesn't want to muddy the relationship between them by becoming friends. Theoretically, if one becomes a friend, they leave themselves open to emotional injuries or other forms of bad blood. If they just keep to themselves and allow the fence and only the fence to mediate their relationship, then there is no problem. There can't be a problem.
You can tell the differences between their personalities in the comparison to apple orchards and pine trees. The speaker's "apple orchards" are ever-changing. They go through the seasons and change their attitudes. The bear sweet, inviting fruit. By contrast, pine trees are deciduous and they are needled, making them uninviting.
He is all pine and I am apple orchard.
My apple trees will never get across
And eat the cones under his pines, I tell him.
In the end, the speaker still doesn't understand the concept, and he thinks about asking: 'Why do they make good neighbors?' He won't ask this question however, and the wall will remain a barrier between the two pieces of land.