The speaker says that the grass cutter ("mower") "loved" them, which is why he (or she) didn't cut them down.
All the same, the mower wasn't doing anyone any favors. The flowers, the speaker realizes, were not left for him, or anyone else ("not for us") (28). Nor were they left as any kind of reminder of the mower. He wasn't interested in "draw[ing] one thought" from anyone about himself (29).
So, was this mower just really awful at cutting grass? Was he or she in a hurry to get done?
Nope—it turns out that the flowers were left intentionally, but it was due to the mower's "sheer morning gladness at the brim" (30).
In other words, the mower was so filled up (the "brim" is the tippy-top of a cup or bowl) with happiness in that moment that the flowers were spared.
Aww—that sure was nice of that person (unless, of course, you're allergic to butterfly weed).
The butterfly and I had lit upon, Nevertheless, a message from the dawn,
That made me hear the wakening birds around, And hear his long scythe whispering to the ground,
And feel a spirit kindred to my own; So that henceforth I worked no more alone;
The flowers are more than just flowers, though. The speaker sees them as a kind of metaphorical message. It's like getting flowers delivered to you from out of the blue. The speaker says that both he and the butterfly are encountering this message.
So, who's the sender? Does someone have a crush?
Well, like any good Facebook status, it's complicated. The speaker claims that the message is coming from "the dawn," but he's really just using metonymy here to mean the mower, who did his or her work in the dawn.
What that message is, exactly, is not made explicit. Even so, it seems to have a profound effect on the speaker.
He now hears things, like the birds waking up around him (naturally) and the mower's scythe—hold up a second. We know that the mower and the scythe are no longer chilling in that field, but it seems like the speaker has some kind of connection to that person now.
The speaker's imagining the sound of the blade moving through the grass, which is personified to be "whispering to the ground" (34).
The speaker feels a kind of connection to the mower at this moment, a "spirit kindred to [his] own" (35).
You know what? The speaker doesn't feel so darn lonely anymore. Sure, he's got that butterfly for company, but it has more to do with his recognition of the mower's humanity in leaving the flowers behind.
Those flowers show that someone was in the field before the speaker and, more importantly, that person made a decision to let them be, instead of just mowing them down. That resonates with the speaker. He feels connected to the mower, now.