Most Elizabethan love poems are about lack. The lover pines, the beloved refuses, and misery gets polished into elegant verse. Sir Philip Sidney's song does something stranger and happier: it announces that love is already settled, and settled fairly. Two people have swapped hearts, and neither has been cheated. The beloved is "he," so the pronouns suggest a woman's voice, and that alone nudges the poem away from the usual picture of a man begging a cold lady for mercy.
The central image is a business deal. The vocabulary is commercial: "Just Exchange," "one for the other given," "bargain driven." The capital letters on "Just Exchange" make fairness feel almost like a law the lovers have obeyed. There is a pun in "I hold his dear," which means both "I treasure his heart" and "I value it highly," as a merchant might. The speaker is delighted with her side of the trade ("There never was a better bargain driven"), and the cheerful boast makes tenderness sound shrewd, as if loving well were also a clever piece of dealing.
The second quatrain shows how the trade works in practice, and the grammar begins to knot itself. "His heart in me keeps me and him in one." "My heart in him his thoughts and senses guides." Hearts, pronouns and possessives pass back and forth so quickly that it is hard to say whose is whose. That confusion is the point. The sentences enact the merging they describe. The explanation for the mutual affection is almost childlike: "He loves my heart, for once it was his own," and "I cherish his, because in me it bides." Each person loves what is lodged inside them, and each is loved for the same reason.
The third quatrain brings in an older convention, the wound of love. Poets often said that love enters through the eyes and strikes like an arrow, so "His heart his wound received from my sight" is familiar territory. What is unusual is where Sidney takes it. The pain does not stay with the one who was hurt. "My heart was wounded with his wounded heart," she says, and the hurt that "did light" on him "did smart" in her. The conventional lover suffers alone, whereas here suffering is shared and so becomes a kind of closeness. "Smart" is a good, sharp word for it, a small stinging ache rather than a grand tragic one.
The form supports all this. The poem is a sonnet of fourteen lines in iambic pentameter, rhymed *abab cdcd efef gg*, the pattern later associated with Shakespeare's sonnets. The rhymes are plain and sturdy (*given/driven*, *sight/light*, *heart/smart*), and they keep the tone close to speech. Even the *one/own* pairing is a rhyme that works better on the page than for the modern ear. The steady, balanced rhythm fits the label "song," and it mirrors the evenness of the exchange. Almost every line divides neatly into two halves, "I hold his dear, / and mine he cannot miss," as if the lines themselves were being weighed on scales.
The final couplet gathers up the story: "Both equal hurt, in his change sought our bliss." "Change" is a loaded word. It hints at exchange, and also at the fickleness love poets usually fear. Here the sense of changing is turned into a promise of constancy, since the only change is the swap that made them one. Then comes the neatest move. The last line repeats the first, "My true love hath my heart and I have his." Nothing in it has altered, yet it now carries everything the poem has explained. The poem ends where it began, a closed circle, which suits a love with nothing left owing on either side.
That is what makes the poem worth reading. It takes the stock materials of Renaissance love poetry, the heart, the wound and the gaze, and uses them to describe a relationship that is mutual rather than hopeless. Its clearest achievement is the conceit of trade, which gives an abstract feeling a concrete shape. It is clever without being cold, and the repeated word "heart," which turns up eight times in fourteen lines, keeps the argument warm.