The poem's personified creatures suggest that the natural world isn't just beautiful, but friendly—and artistic.
In the hottest part of the summer, when the birds are so "faint" they can't keep singing, one voice remains steady: the Grasshopper's. Like a lot of fictional grasshoppers, this one is a musician and a pleasure-lover; his delight in "summer luxury" persists even through the heat, which he sees as an excuse to relax under "some pleasant weed."
More than that, though, he's the voice of the "poetry of earth": it's his constant song that makes the speaker declare that such poetry is "never dead." If that's so, the "poetry of earth" isn't just about nature being lovely, but about nature enjoying itself—literally. The personified Grasshopper's sense of "fun" and pleasure suggests that the earth itself takes constant joy in its own being, even on the stillest, sweatiest summer day.
The weather, too, becomes an artist in lines 10-11, where the speaker describes how "the frost / Has wrought a silence." The word "wrought" here presents the frost as an artisan, crafting silence as one might craft a silver necklace. The Cricket's winter song, cutting across that silence, plays the same role as the Grasshopper's, suggesting a perpetual note of life and happiness running through "the frozen time" (as Keats called it in another poem).
All these personifications thus evoke a living world full of creative delight—a delight, the poem suggests, that remains constant even through uncomfortable times.