Shakespeare’s speaker is caught in a love triangle that hurts more than a simple betrayal. He admits that losing the woman to his friend isn’t his deepest grief — what truly wails is that she now possesses his friend, a loss in love that cuts closer. He tries to excuse them both, arguing that the friend only loves her because he knows the speaker loves her, and she merely allows this to please the speaker. It’s a twisted, self-centered logic that reveals how desperate he is to make sense of the pain.
That desperation reaches its height in the final couplet. He imagines a joy: since he and his friend are, in spirit, one, then her loving the friend means she loves only him. He immediately calls this “sweet flattery” — he knows he’s deluding himself. The mood is wry and heartbreaking, full of the kind of cleverness that masks a wound too deep to face directly. What remains is the ache of double loss, barely wrapped in a lie.