I looked for him behind an isle of trees; I listened for his whetstone on the breeze.
But he had gone his way, the grass all mown, And I must be, as he had been,—alone, (5-8)
This seems to describe a game of hide-and-seek, in which our speaker comes up a big, fat loser. He can't find the mower anywhere, which leads him to reflect on how his isolation is something that everyone must experience—bad times.
'As all must be,' I said within my heart, 'Whether they work together or apart.' (9-10)
At first, not even working together is—enough in the speaker's view—to bridge the gap between two people. In one way, we can see the logic here (depressing as it may be). How can we ever really, truly be connected to someone if we all have our own individual experiences?
I thought of questions that have no reply, (19)
We'd love to know what those might be. The speaker never gives them up, but we based on the context we can sure hazard a guess: "Is true human connection possible?" "Is isolation an inescapable part of the human condition?" "If a tree falls in a forest, how many Robert Frost poems could I print on the paper it gets turned into?"
And feel a spirit kindred to my own; So that henceforth I worked no more alone; (35-36)
Luckily for our speaker (and us, too), there is a resolution to the speaker's isolation. Tellingly, he finds it in his work. As he does his own job of turning the grass, he's able to end his isolation by finding a connection to the mower's work (and kindness to flowers) that came before him.