The theme is the boundaries that separate us, and whether they are needed or not.
Explanation:
The premise of this poem is a neighbor and the speaker repairing the wall that splits their land, and the speaker is unsure as to why the wall serves any purpose. He argues that there are no cows to keep inside and nothing to keep outside- all the beautiful trees are in their land.
This has a deeper meaning, which is the idea that we sometimes aren't aware of what we are "walling in" or "walling out" in terms of social and mental boundaries.
