Paraphrase the last stanza of Thomas Hardy's "On the Departure Platform." —‘And why, young man, must eternally fly A joy you’ll...

Here's a paraphrase:

When you truly love her, why does an elation that you want to replicate have to be gone forever? I cannot explain why nothing recurs exactly.

It's good to note that the final stanza is not told from first person point of view like the rest of the poem.  Hardy switches speakers from the young man in love to an omniscient, wiser observer.