The speaker is looking back on an old pain—a time when the person he loves treated him badly. Now that he has, in turn, hurt that same person, that earlier wound becomes something of a strange comfort. He admits his own guilt without excuse; unless he were totally unfeeling, he has to bow under the weight of his transgression. He imagines the hell his beloved went through because of his unkindness, and regrets that he didn’t learn from his own past suffering to offer comfort sooner. The couplet shifts the idea to a kind of emotional bookkeeping: your old fault becomes a price paid, and my fault pays for yours. Forgiving each other, in this view, cancels both debts. The mood is tender and self-reproachful, but with a quiet hope that shared pain can lead to mutual healing. It’s a poem about the way our own failings can teach us to forgive the failings of others.
Sonnet 120: That you were once unkind befriends me now — Summary
That you were once unkind befriends me now, And for that sorrow, which I then did feel, Needs must I under my transgression bow, Unless my nerves were brass or hammer'd steel. For if you were by my unkindness shaken, As I by yours, you've pass'd a hell of time; And I, a tyrant, have no leisure taken To weigh how once I suffer'd in your crime. O! that our night of woe might have remember'd My deepest sense, how hard true sorrow hits, And soon to you, as you to me, then tender'd The humble salve, which wounded bosoms fits! But that your trespass now becomes a fee; Mine ransoms yours, and yours must ransom me.