Final answer:
The theme that best describes 'Mending Wall' by Robert Frost is that people often build up barriers out of habit. The repeated rebuilding of the wall depicts the continual effort needed to maintain boundaries in relationships and questions the necessity of these barriers.
Step-by-step explanation:
One of the central themes in Robert Frost's poem 'Mending Wall' is the notion that barriers in relationships can be both necessary and detrimental. Frost uses the annual ritual of two neighbors rebuilding a stone wall to explore this complexity. The wall serves as a physical and metaphorical barrier, prompting the speaker to question the need for such divisions. While his neighbor insists that 'Good fences make good neighbors,' the speaker contemplates the walls people build to define their personal space and boundaries. The poem suggests that while walls can foster individuality and privacy, maintaining them requires effort and sometimes the reasons for their existence are not clear or justified, especially when there is no apparent reason for keeping the neighbors apart, symbolized by the absence of cows that might stray.
The theme that 'people often build up barriers out of habit' is the one that most closely aligns with Frost's message in 'Mending Wall.' The neighbor's adherence to his father's saying reflects a reluctance to change and a comfort in tradition, even when the reasons for those traditions may no longer be relevant.