Keats begins by addressing a star, then almost at once takes back half of what he has said. "Bright star, would I were steadfast as thou art" sounds like a wish to become the star, until the next line interrupts: "Not in lone splendour hung aloft the night." What he envies is the star's constancy, not its isolation. The whole poem lives in that split between the longing for something that lasts and the fear that lasting might mean being cold and alone.
The first eight lines, one long sentence, build the case against the star's life, and they do it beautifully. The star is a "patient, sleepless Eremite", a hermit, watching with "eternal lids apart". The detail is quietly unsettling, because here constancy means never being able to close your eyes. From that height the star sees water at its "priestlike task / Of pure ablution", washing the "human shores", and snow lying like a "mask" on mountains and moors. The vocabulary is religious, ritual and clean, and the earth becomes a ceremony rather than a place where anyone lives. "Mask" is the word that nags: snow covers things over, and the star sees only surfaces. Even "human" feels pointed, a reminder that the watcher looks at us without belonging to us.
Then comes "No—". The blunt monosyllable and the dash make the pivot of the poem. The rhyme scheme (abab cdcd efef gg) is the Shakespearean one, yet the argument turns at line nine, where the sonnet tradition of Petrarch would place its turn, so the poem seems to borrow its shape from one pattern and its thinking from another. What follows keeps the star's steadiness but moves it from sky to body: "still steadfast, still unchangeable, / Pillow'd upon my fair love's ripening breast." "Pillow'd" is soft and warm, the opposite of "aloft". But "ripening" smuggles change into a passage about unchangeableness, since ripe things keep on moving toward something else. The poem never quite settles this contradiction, and it is more honest for that.
The rhythm helps. The lines are iambic pentameter, and the steady beat lets the later lines rock gently. "To feel for ever its soft fall and swell" rises and falls as it is read, with its soft f, s and l sounds, like the breathing it describes. "Awake for ever in a sweet unrest" is an oxymoron that sums up the wish: he wants stillness that is nonetheless alive. The word "still" works hardest of all. It means motionless, it means "yet", and it means "always". By line thirteen it is doubled, "Still, still to hear", and the repetition itself sounds like a pulse. Notice, too, how the verbs have shifted to the senses, "feel" and "hear", where the star could only watch and gaze.
The couplet closes the poem like a door, with breath rhymed against death. "And so live ever—or else swoon to death" is almost all single syllables, slow and even, and the dash lets the alternative drop in without warning. If this moment cannot last forever, he would rather end. "Swoon" is a slippery word, since it can mean fainting with pleasure as well as dying, so ecstasy and extinction sit inside one verb. The poem ends on a wish that sounds like a demand, but the "or else" suggests the speaker knows how impossible it is.
That is the poem's real interest. Many poems about love reach for the stars as symbols of the eternal. This one looks at the star's immortality and turns it down as a poor bargain, because eternity without touch, sound and warmth is not worth having. Permanence matters to the speaker only if it can be felt, in one person's breathing, in the dark. The poem asks whether a moment can be made to last without ceasing to be a moment, and it does not pretend to have an answer.