Frost is emphasizing that his neighbor is a creature of habit. The neighbor doesn't want to change any of the actions he has taken over years and years of doing them. He has become comfortable with a pattern of setting a wall between his property and his neighbors. The neighbor chooses to keep that separation in place in spite of the fact that the wall is unnecessary where it stands. Many times people who are reluctant to change rely on traditions of the past as confirmation that there is no need to change. For the neighbor, his father's belief that, "Good fences make good neighbors," emphasizes his own belief that if it was good enough for his father, it is good enough for him. Frost would rather that the neighbor thought for himself and step away from his dogmatic attachment to traditions of the past. Because the neighbor chooses to "move in darkness" Frost feels his neighbor has missed an opportunity to do something different, to let go of his stodgy fondness for an unneeded barrier between the two of them.
Frost's neighbor repeats the phrase "Good fences make good neighbors" twice:
1. Frost tells his neighbor that a man-made wall is not necessary because their boundaries are naturally and clearly demarcated as Frost grows apple trees in his plot of land whereas pine trees are growing in his neighbor's. But his neighbor refuses to accept Frost's logical argument. He is simply incapable of reasoning because he is blinded by the dogma and tradition of what his forefathers have drummed into him over the years.
2. Having failed to reason logically and convince his neighbor that the wall is not necessary he hopes that his neighbor will atleast intuitively understand that "Something there is that doesn't love a wall/That wants it down." Every Spring, Frost and his neigbor meet to rebuild the broken wall. This is an annual ritual which both of them have performed routinely for many years. Frost is now an enlightened person who clearly understands the futility of this annual ritual. Frost hopes that his neigbor will also instinctively gain this same knowledge and understanding. But no such flash of inspiration or intuition can penetrate into his neighbor who resembles a stone age savage completely blinded by dogma and tradtion and who smugly repeats,"Good fences make good neighbors."
Frost repeats this cliche twice to foreground the fact that logical reasoning and intuitive inspiration were both unsuccessful in enlightening his neighbor.