What does Frost seem to be saying about humankind in the poem 'Mending Wall'?
Frost seems to be implying that humans are carelessly destructive, divisive and bureaucratic.
The first indication is from the hunters:
Where they have left not one stone on a stone,
But they would have the rabbit out of hiding,
To please the yelping dogs.
The "yelping dogs" may suggest that this is an allegory for poor decisions made to please an emotional mob. The hunters destroyed something that took care, skill and effort to create, simply because it was in the way of a minor victory (note that the hunters are not said to have caught the rabbit, only to have chased it out of hiding). If we were to consider this in terms of, say, chess, this would clearly be a very poor trade in value.
The narrator goes on to discuss rebuilding the wall with his neighbor, and wondering why they have a wall at all. The neighbor repeats a mantra that "good fences make good neighbors", but the narrator wonders why this should be so. Nevertheless the saying itself seems to carry more importance to the neighbor than any real truth, and he repeats the saying as if it is holy, and he doesn't dare question it.
The narrator also remarks on the difficulty of building the wall, and how it is
Oh, just another kind of out-door game,
One on a side. It comes to little more:
meaning that ultimately the wall is a silly thing based on silly rules when it is really scrutinized.
Finally, the poem begins with, and carries an underlying tone, that nature itself does not like walls, because it gradually breaks them down.
Keep in mind that all of this is from the perspective of the narrator, who has a disdainful view of other people; it is possible that this view, itself, can be reflected upon in light of the topic question. See the enotes link below for more insight from this angle.
Frost's poems in general have been over-analyzed, as though his main motive was to steep great human insights into allegorical stories. There is considerable evidence, however, that he was much more interested in drawing sketches of New England life, and he can be seen as a short-story writer who is relating specific incidents in his (or the narrator's) life. His work taken together is really a portrait of a New England rural/agricultural life gradually fading into modern technology. The "symbolism" is this poem lives not so much in the poet's mind (and with modern criticism, the author's intentions are of minor importance) as in the readers' tendencies to "make something" of the story. While from a Jungian standpoint such universals as walls, hunters, a personified Earth, etc. (and we can expand this remark to include woods, dark evenings, paths not taken, etc.) draw us to universal "truths," to read these poems as puzzles is to miss the smooth rhymes and flows of the actual texts, and to abuse the visual and aural beauty of his creations, the lines and textures, the "craft." True, Frost is an astute observer of human conduct, but he is not first and foremost a Symbolist.
What I always got from this poem is that humans are finding ways to distance themselves from others. The poem says that "Good fences make good neighbors" meaning that as long as you stay away, no trouble is caused, but it also means that we don't contact each other or sit down and chat. "He will not go behind his father’s saying...". The man keeps wanting to rebuild the wall because that's what has always been done, even though there is no conflict between the neighbors. How I interpret it, Frost is saying that people are accustomed to keeping to themselves and find it better to distance themselves than to potentially cause war with each other.
By keeping people away from other people, there is less conflict and less potential for conflict.