Final answer:
In Robert Frost's "Mending Wall," the natural force that "doesn’t love a wall" is ice, which causes the wall to decay by pushing the boulders out of place.
Step-by-step explanation:
Robert Frost's poem "Mending Wall" explores the thematic element of decay and maintenance, not only in physical structures such as walls but also in human relationships. The phrase from the poem, "Something there is that doesn’t love a wall," describes a natural force, which in this case is ice.
As the ice forms in the ground, it causes freezing and thawing, which leads to the ground swelling and thus pushing the boulders up and out, creating gaps in the wall. These gaps are significant enough for two people to pass abreast, suggesting a considerable weakening or breaking of the structural integrity of the wall.
This reflects the poem's central theme that there are elements, whether natural or inherent in human nature, that resist the barriers represented by the wall, evoking ideas about the nature of boundaries and separation between people.