Very timely. BTTT.
Its a tricky little poem. The narrator/poet/Frost is not the one who says the oft-quoted line, "Good fences make good neighbours." Instead, the line is left to the person on the other side of the wall, who has had this passed down to him from his father.
In fact, the 'narrator/poet/Frost' appears not to believe the intent of the key line and so he plays Devil's Advocate and asks, "Spring is the mischief in me, and I wonder If I could put a notion in his head: Why do they make good neighbours?". This question never seems to be answered in the poem.