We know there is something, but we don't know what. The wall is a symbol of strength as we are told that it "spills the upper boulders" and "makes gaps even two can walk abreast". The forces that send out the "frozen ground-swell under it" are set on destroying it, yet these forces are never revealed. Frost uses the line twice in the poem to remind us that something somewhere does not like a wall.
The neighbors use the wall as a gathering place to meet and share an outdoor experience. They share the common nuisance of hunters that have damaged the wall. They struggle to mend the wall together and cooperate on locating it exactly on the property line. The difficulty they share is shown when Frost says, "We have to use a spell to make them balance: 'Stay where you are until our backs are turned!' We wear our fingers rough with handling them.". This common goal is "just another kind of outdoor game". Frost is sharing the outdoors and enjoying the wall.
While the neighbors are mending the wall, the wall is acting as a separation to the men. Frost says that "And on a day we meet to walk the line and set the wall between us once again. We keep the wall between us as we go. To each the boulders that have fallen to each". The wall has defined the space and dictated the realm of each man. They are together, but still divided. They are alone but not lonesome.
Still, Frost questions the need for walls. His neighbor insists that "Good fences make good neighbors", but Frost questions the need. He acknowledges the differences with his neighbor when he says "He is all pine and I am apple orchard". Yet Frost insists the apples will not invade the pine. Walls were meant for cows and "here there are no cows". Frost is playing the devils advocate and realizes that "Spring is the mischief in me". He is fond of the wall for inexplicable reasons.
He states that if he was to build a wall he would need to know "What I was walling in or walling out". Frost doesn't indicate that he has any hardened opinion on the wall. Yet, the feeling that something wants it to come down haunts him. He muses that "I could say 'Elves' to him, But it's not elves exactly". For Frost there is a spiritual force that is working against the building of walls. Yet, he makes no contradiction that they make good neighbors.
Frost is saying that walls are a necessary part of life that is continually challenged. The walls that separate us also define us. Without the wall there would be no apple, no pine, and no spring mischief. Frost's reservations about the wall are due to the separation that they impose on others. He does not take them personally and only wonders that "to whom I was like to give offence" by building a wall. The wall, with all its strength and spiritual opposition, is a mere inconvenience for Frost.
The uneasy tension that exists between the lines "Something there is that doesn't like a wall" and "Good fences make good neighbors" is Frost's way of questioning where our space ends and another's begins. In the poem, Frost is rather indifferent about the existence of the wall. Yet, he takes the time and energy to rebuild it every spring out of respect for his friend and neighbor. In doing so, Frost has told us the value of a wall. It is the respect for other people's space and the need for constant