In this poem, a man is seeing his beloved off at a train station and watches her disappear in her "soft white array." The man knows they will meet again, as they have made plans to do so, but he muses that she will never quite be the same again, even if she appears in the same soft white array.
In the last stanza, another person, either a friend with whom he has been talking or a stranger who overhears him commenting to himself, asks the man why he thinks his beloved will never quite be the same again. He asks why must "eternally fly," meaning leave forever, a joy that will repeat if he loves her? What the questioner means is, why should your beloved change if you love her? He may be thinking of Shakespeare, who wrote sonnets about the beloved never changing. This lover, however, is more prosaic and not contemplative: He merely says, in the last line, that nothing happens the same way twice, and he adds that he doesn't know why. He expresses what he has experienced: that moments are unique and fleeting. The poem captures both the fleeting moment and then the final comment that it will never come again in quite the same way.