"Why do [good walls] make good neighbors? Isn't it
Where there are cows? But here there are no cows.
Before I built a wall I'd ask to know
What I was walling in or walling out,
And to whom I was like to give offense.
Something there is that doesn't love a wall
That wants it down!..." (Ln. 29-35)

The mood of the poem is lonely. The author feels lonely because he won't talk to his neighbor, only because of a wall that his neighbor and him are building and reinforcing themselves. The loneliness of the author causes the audience to feel just as lonely.

Mood