Shakespeare opens by asking a question that sounds almost like a riddle: why does a person who loves music listen to it with sadness? "Sweets with sweets war not, joy delights in joy," the speaker reasons, so something must be wrong. The young man he addresses seems to take pleasure in what should hurt him, or to be hurt by what should please him. Sonnet 8 belongs to the opening run of the sequence, where the speaker urges a young man to marry and have children. The poem is built to make that argument through music rather than through plain instruction.
The central idea is that harmony needs more than one. The speaker suggests the man's sadness at music is really a rebuke. If "well-tuned sounds, / By unions married" offend his ear, it is because they "sweetly chide" him for being one voice when he could be part of a chord. The phrase "confounds / In singleness the parts that thou shouldst bear" is neatly double-edged. "Parts" are the separate voices in a musical arrangement, but they are also the roles a person might play in life. Reading "bear" as a hint at bearing children is an inference, though a natural one given the poems around it.
Shakespeare makes this idea physical in the third quatrain. One string, a "sweet husband to another," sets the others vibrating, "Strikes each in each by mutual ordering." The image of strings answering one another makes harmony sound like a conversation, and the language of marriage runs through it: "unions married," "husband," and then "sire and child and happy mother." Music and family become the same thing. Three different voices "all in one, one pleasing note do sing." The heavy repetition of "one" in that line enacts the blending it describes.
The couplet delivers the point with a twist. The song is "speechless," yet it speaks. Its "many" voices "seem one," and what it says is the sentence that closes the poem: "Thou single wilt prove none." The word play on "one" and "none" is the sharpest moment in the sonnet. In Shakespeare's English the two words were close in sound, which makes the rhyme feel like a trap closing. Being "single" means being unmarried, but it also means being merely one. In the poem's logic, one is not a smaller version of a chord but nothing at all. It is also clever that this verdict comes from the music itself, not from the speaker. The poet steps back and lets the instrument accuse the listener.
The form supports the argument. This is a Shakespearean sonnet, with three quatrains rhyming ABAB CDCD EFEF and a final couplet. Each quatrain takes a different step: the question, then the diagnosis, then the illustration. The couplet draws the conclusion. The rhythm is iambic pentameter, but Shakespeare loosens it at the start. "Music to hear" falls on a stress at the very first syllable, which pushes the opening forward and gives the question the feel of a spoken challenge. Several lines end on an extra unstressed syllable ("sadly," "gladly," "another," "mother"), a soft, falling finish that suits the gentle teasing tone. The rhyme "ordering / sing" is only approximate to a modern ear, so the quatrain's pairing has a slight looseness, as though the strings are not perfectly tuned until the couplet snaps them into place.
The sonnet is significant because it shows how persuasion can become poetry. A flat instruction to marry would be forgettable, but Shakespeare builds a small world of sound in which the argument seems to arise on its own. The poem also contains a gentle irony. A single speaker is lecturing a single listener about the dangers of being alone, yet by the end his one voice has somehow produced a chord of father, mother and child. Nothing in it is angry or harsh. The pressure is affectionate, almost playful, and that makes the final line land more firmly. The poem also leaves a question open for readers to test. Is it true that a person on their own "proves none," or is that the kind of overstatement a clever persuader makes in order to win?