Envy is an odd feeling to find in a winter poem, but Keats's December is full of it. The speaker looks at a bare tree and a half-frozen brook and calls them "too happy," as though their bleak season were a piece of luck. The reason comes in the third stanza. Nature forgets, and people can't. The tree does not mourn its summer leaves, and the brook does not pine for the sun, so both are free to begin again. Human beings carry their past with them, and remembered joy becomes a form of pain. The poem is a small, song-like lament that we cannot be as forgetful as a tree.
The first two stanzas build their case through personification. The branches "ne'er remember / Their green felicity," and the word "felicity" gives summer leaves the dignity of a lost happiness. Winter ought to be the tree's enemy, yet Keats makes its attacks sound harmless. The north wind is a "sleety whistle," more a nuisance than a blow, and the "frozen thawings" that might "glue" the buds shut fail to stop them "budding at the prime." The verb "glue" is sticky and almost childish, which helps it feel unthreatening. The brook is treated more playfully. It does not recall "Apollo's summer look," as if the sun were a handsome god it once admired. Instead it enjoys "a sweet forgetting." The ice that stops its "crystal fretting" is presented as a mercy, a calming of nerves, and "petting," meaning sulking or peevish grumbling, makes the brook's old restlessness sound like a mood it has outgrown.
The form supports this lightness. The lines are short, mostly seven or six syllables, and the seven-syllable ones end on an unstressed syllable ("December," "remember," "undo them"). That gives them a skipping, music-box lilt. Each stanza has the same pattern: four lines of alternating rhyme, then three lines on a single rhyme, then a final line that is shorter and indented. The triple rhymes ("undo them," "through them," "glue them"; "forgetting," "fretting," "petting") are almost comic in their jingle, and they make winter feel gentle. The closing lines of the stanzas, "prime," "time," and "rhyme," also chime with one another across the whole poem. Each lands like a small full stop.
Then the third stanza breaks the spell, and the "Ah!" is a sigh. The speaker turns to "a gentle girl and boy," and the question that follows, "were there ever any / Writhed not at passed joy?", answers itself. "Writhed" is a bodily word, much harsher than anything in the first two stanzas, and it shows what the tree and brook are spared. The last four lines are the hardest to read, and the knottiness seems deliberate. "The feel of not to feel it" names the blessed numbness the tree enjoys, a feeling of not feeling. Humans have "none to heal it" and no "numbed sense to steel it," meaning no cure for the memory and no toughening of the nerves against it. The final line, "Was never said in rhyme," is a teasing paradox, since the poem has just said it in rhyme. One way to read it is as an admission that this blank, painless state can't really be captured in words, so the poem only gestures at it.
What makes the poem worth reading is how much it fits into so little space. The stanzas look like a nursery song, yet they hold a sharp thought about memory, happiness and loss: that the ability to remember joy is exactly what makes losing it hurt. That concern with pleasure and pain living side by side is one readers of Keats will recognise. In his "Ode to a Nightingale," for instance, "fret" appears again as a mark of human weariness. Here the idea is simpler and quieter. The speaker does not want a grander life than the tree's, only its easy winter, and the cheerful music of the poem makes that wish sadder.