The speaker asks what she owes someone who loved her deeply and for a long time, and the answer is a cold one: nothing much. That devoted love never lifted her spirit or stirred her heart to sing. It was real, but it left her unchanged.
Then the second stanza turns on the word "But." The man she loved, who did not love her back at all, gave her something far greater. Through the pain of that unreturned feeling, she glimpsed something larger than ordinary life, a "little open gate" in "heaven's wall." The image is small and quiet, yet it suggests a rare moment of wonder.
The mood is wistful and a little bittersweet. There is an unsparing honesty here, since the speaker admits that being loved did less for her than loving did. The poem also hints at a mild guilt toward the faithful lover, though she doesn't apologize. Her debt, she says, is to the one who gave her nothing, and that irony gives the short poem its sting.