What lines are repeated in the poem "Mending Wall"?

The lines “something there is that doesn’t love a wall” and “good fences make good neighbors” are repeated.

Repetition is used in poems to add emphasis and highlight significant themes.  In this case, the poem is about a pair of neighbors who disagree on whether there should be a wall between their farms.  One thinks that it is better to keep your neighbor at a distance, and the other does not see the point of the wall and prefers more contact between neighbors.

The first and last lines of the poem are parts of the repetition.  Each of these are related to the main theme of the poem, which is that we put up walls against other people because we feel that we are better off keeping others at a distance.

The speaker does not like maintaining the wall between the two farms.  He feels that it is unnecessary, using the fact that the wall seems to crumble as proof that it should not be there.

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.

The neighbor, on the other hand, carefully reconstructs the wall each year.  The speaker wants to be a good neighbor by getting to know his neighbor, but the neighbor wants to be a good neighbor by avoiding his neighbor.  It is two different approaches to the act of coexisting with others.

The neighbor has a policy that seems to support keeping the wall intact.

He moves in darkness as it seems to me,

Not of woods only and the shade of trees.

He will not go behind his father’s saying,

And he likes having thought of it so well

He says again, ‘Good fences make good neighbors.'

The two positions seem to have no middle ground.  The speaker feels that since neither of them have livestock, there is no reason to have a wall.  The neighbor prefers tradition.  He likes to keep himself isolated from his neighbors.

It should be noted that both neighbors work to rebuild the wall.  This is a metaphor too.  Although the speaker's neighbor prefers the wall, keeping it there is a collaboration between the two of them.  When others shut us out, we need to cooperate in order for them to really keep us out.