Forgiveness rarely sounds as suspicious of itself as it does here. The speaker sets out to comfort someone who has wronged him ("No more be griev'd at that which thou hast done"), but by the end he has turned the same scrutiny on himself and found himself guilty. What looks like generosity becomes a confession: he is so eager to excuse the offender that he becomes part of the offence.
The first quatrain is a small parade of consoling proverbs. Roses have thorns, fountains have mud, clouds and eclipses stain the sun and moon, and a canker lives in the sweetest bud. The argument is that nothing beautiful is flawless, so why expect it of a person? Yet the images are less comforting than the speaker wants them to be. A thorn sits beside the rose, but a canker, a grub that eats a flower from within, lives *in* the bud. Shakespeare sets "loathsome" against "sweetest" in the same line. The list slides from the merely blemished to the truly corrupt, as if the speaker's language cannot help noticing the rot he is trying to shrug off. Line 2, "Roses have thorns, and silver fountains mud," is almost entirely one-syllable words. It has the flat ring of a proverb, the sort of thing people say to make a problem smaller.
At line 5 the poem turns on itself. "All men make faults, and even I in this" reveals that the speaker's fault is the excusing itself. "Authorizing thy trespass with compare" admits that the comparisons of the opening were special pleading, a way of giving the offence official approval. Then the self-accusation piles up in a run of verbs: "corrupting," "salving," "Excusing." The quatrain ends on a piece of cold arithmetic, "Excusing thy sins more than thy sins are," where the excuse has grown larger than the wrong it covers.
The third quatrain turns the mind into a courtroom. "Trespass" has already appeared, and now come "advocate," "adverse party," "lawful plea," "commence." One man plays every role. He is the injured party and also the defence lawyer, and he starts a case against himself on top of that. The dashes around "Thy adverse party is thy advocate" make the line feel like a gasp at his own absurd position. There is also a pun in "sensual fault" and "sense." The offence belongs to the appetites, and he answers it with "sense," meaning reason, but he uses that reason to defend rather than to judge. His image for the whole muddle is "civil war," a conflict fought inside one country. Love and hate occupy the same ground, and neither can retreat.
Formally the poem is a tidy Shakespearean sonnet, in iambic pentameter, rhyming ABAB CDCD EFEF GG. That order sits against the emotional disorder, and the couplet is where it shows. "That I an accessary needs must be" twists its word order as though the admission were being forced out, and "needs must" carries the sense of compulsion. The last line, "To that sweet thief which sourly robs from me," packs its contradiction into two words, "sweet" and "sourly." The loved one is delightful and damaging in the same breath. The poem never says what was stolen, which keeps our attention on the speaker's state of mind rather than on the event. The closing rhyme of "be" with "me" is neat too. The victim gets the final word, even as the logic makes him an accomplice.
The poem matters because it is honest about how affection bends judgment. It does not give us noble forgiveness, nor a clean break. It gives us a mind that sees the situation clearly and still cannot stop making allowances, and that is aware of the contradiction. In fourteen lines it moves from comforting someone else to cross-examining the self, a reversal many readers will recognise from times they have talked themselves into excusing someone they cared about.