Shelley picks a quarrel with Shakespeare before the first line is over. The words in quotation marks, "food of Love", are borrowed from the opening of *Twelfth Night*, where Duke Orsino asks for music to feed his lovesickness ("If music be the food of love, play on"). Shelley's reply is a blunt "No", placed at the very start of the line, before he even says who he is talking to. In three lines he takes the old idea apart and rebuilds it.
The argument turns on one small word, "Unless". At first Shelley seems to deny that music nourishes love at all. Then he allows an exception: Love might feed "upon its own sweet self". That exception swallows the rule. Love needs no outside supply, so music cannot be its food. The last line goes further: love goes on feeding "Till it becomes all Music murmurs of." The usual order is reversed. Music is no longer the cause of love but a faint report of it, and love is the thing itself, which music can only murmur about. The verb matters here. A murmur is soft and indistinct, a kind of half-telling, so music is made to sound like a pale echo of what it describes.
The poem is built on a single metaphor, eating. "Food" in the first line becomes "feeds" in the second, and the image is pushed until it turns strange. A love that feeds on "its own sweet self" is self-sufficient, but it is also a little unsettling, since something that eats itself must be using itself up. "Sweet" makes the picture pleasant, but the poem does not say whether this self-consumption is a triumph or a danger. Judging by the tone, which is light and teasing, Shelley probably intends a celebration, though the image leaves room for a shadow.
The sound of the lines supports the sense. Each has ten syllables and falls into a fairly regular iambic pattern, which gives the thought a calm, certain tread. The first line opens with two heavy beats, "No, Music", so the denial lands hard before the rhythm settles. The rhyme is unusual. "Love" and "of" nearly rhyme, and they stand at the start and end of the poem, while "self" sits unrhymed between them. Love is wrapped around the idea of the self, as in the thought itself. The sentence runs on from the second line into the third without a pause, as if love's appetite were carrying the reader along. The consonants also play a part. "Music murmurs" hums with repeated m sounds, so the line makes the murmur it describes, and "sweet self" has a soft hiss that sounds almost like a secret.
The poem's final word is "of", a small preposition that leaves the sentence hanging. We expect an object to follow, something music murmurs *of*, but the poem stops. The answer is already in the poem, because it is Love, the word that began the line before. The ending therefore trails off like a murmur dying away, and it also closes a circle, so that the poem has fed on its own opening. Capital letters and the old-fashioned "thou" make Music and Love into characters, and Music is addressed like a friend who is being politely corrected.
The title calls this a fragment, yet it reads as complete, a thought with a beginning, a turn and an ending. That is part of its interest. It shows a poet answering an earlier writer by quoting him and then quietly changing what the quotation means. It also makes a claim about art that is worth testing. Is music something that produces feeling, or only a hint of feelings we already have? Shelley chooses the second view and puts feeling above the art that expresses it. A reader may disagree, but in only thirty syllables the poem makes the disagreement worth having.