The speaker tells the beloved that he has taken all the speaker's loves and now possesses them, so the beloved has gained nothing new—all the speaker's affection was already his.
Lines 1–6 execute a brilliant rhetorical move. The speaker invites the beloved to 'take all my loves,' then immediately asks: what has he gained? The answer: nothing. Nothing real. The 'true love' was already the beloved's. All the speaker's other loves were false substitutes for the beloved. Therefore, the beloved hasn't stolen anything; he merely reclaimed what was always his. Lines 5–6 drive home this logic: 'Then if for my love, thou my love receivest, / I cannot blame thee, for my love thou usest.' The beloved is using the speaker's love, which is an inanimate possession. Technically, the argument goes, infidelity is impossible when the real love was never actually given to the other person. This is sophistry, but it's sophisticated sophistry—a way to absolve the beloved of guilt by redefining what love 'counts.'
Lines 9–14 shift from logic to emotion. The speaker forgives the 'gentle thief'—a paradoxical epithet that contains both tenderness and accusation. Line 10's 'steal thee all my poverty' is brilliant: 'poverty' can mean both lack of wealth and lack of romantic merit. The beloved steals the speaker's diminishment. Lines 11–12 then make the crucial revelation: the greater grief is not being betrayed by a rival but by someone you love. Hate's injury is cleaner; love's betrayal is more wounding because it violates the sacred bond. Lines 13–14 conclude with an image of seductive evil: the beloved's 'lascivious grace' makes 'all ill well shows'—sin looks beautiful when embodied in the beloved. The final line—'Kill me with spites yet we must not be foes'—admits that the speaker is being killed emotionally yet insists on friendship. It's heartbreaking resignation.
Your best friend sleeps with your ex or your crush. You're devastated. But then you think: well, I was only with that person because I was thinking about you anyway. My heart was already yours. So technically, you didn't steal anything that was truly mine. It's twisted logic, but it's what hurt people tell themselves to survive.