Eight short lines, and already a puzzle: why would a man tell a woman she has nothing to fear from him, while admitting that he is afraid of her? Shelley's speaker addresses a "gentle maiden" and starts with a confession, "I fear thy kisses." Then comes the reassurance, "Thou needest not fear mine." The title, "To --", leaves the addressee blank, so she becomes any beloved, and the poem becomes about a pattern of feeling rather than one named relationship.
The first stanza explains the imbalance through weight. His spirit is "too deeply laden / Ever to burthen thine." "Burthen" is an old form of "burden," and "laden" suggests a ship riding low with its cargo. The picture is of someone already so loaded down that he cannot add anything to another person's life without hurting her. The poem never says what the load is, and that silence is part of its effect. We get the consequence of his heaviness, a gentle self-withdrawal, without its cause. The logic is odd but moving. The more he feels, the less he thinks he should touch her.
The second stanza widens the fear. The first named only her kisses, a single intimate act. Now he fears her "mien, thy tones, thy motion": how she looks, how she sounds, how she moves. The fear has spread from a gesture to her whole presence. Her effect on him is not something she does on purpose. It is simply how she is. The repeated line, "Thou needest not fear mine," now covers his mien, tones and motion, so the same words carry a slightly larger promise.
Then the vocabulary shifts. "Innocent is the heart's devotion / With which I worship thine." Devotion and worship are religious words, and they quietly recast the fear. What he feels may be less like dread than like awe, the kind people feel before something they hold sacred. "Innocent" sounds almost like a plea, a man assuring her that his love has no hidden claim on her. The spirit of stanza one has become a heart here, and the final word of the poem is "thine," so it ends on her rather than on him.
The form supports this humility. Both stanzas rhyme ABAB, but the two kinds of rhyme feel different. "Maiden/laden" and "motion/devotion" are two-syllable rhymes that end softly on an unstressed syllable. "Mine/thine" is a single-syllable rhyme that lands firmly, like a door closing gently. The lines alternate in length too, with long lines of about nine syllables, roughly iambic, followed by short lines of six. The long lines carry the confession, and the short ones deliver the reassurance in plain, brief words. Notice also that "mine" and "thine" rhyme in both stanzas. The sound ties the two people together even while the speaker insists on keeping his distance.
The old-fashioned "thou needest" adds a formal courtesy, as if he were speaking carefully so as not to frighten her. Yet the poem's tone is not entirely settled. He soothes her, but his own fear is never answered. It is the first thing he says, and nothing in the poem resolves it. He asks her to give up her fear while keeping his, which makes the reassurance feel tender and slightly lonely at once.
The poem's interest lies in this compression and in how it reverses expectations. Love poems often urge the beloved to yield, with the speaker as pursuer. Here the speaker is the one who holds back, and the poem presents restraint as a kind of devotion. In a handful of plain words it sets fear against reverence, and heaviness against the lightness he wants to protect in her.