Where were you before you were born? No one knows. Where will you go after death? No one knows either. In between, for a brief stretch, you appear — visible, tangible, here. Then you slip back into the unseen. Krishna asks: in this pattern, what is there to mourn?
Picture a wave rising from the ocean. Before it rose, it was ocean. While it crests, it seems to be its own thing — distinct, individual. Then it falls back and is ocean again. The wave was never separate from the water. It simply appeared for a moment and returned. Living beings are like that — emerging from the unmanifest, shining briefly in manifest form, then dissolving back into the unmanifest.
The word 'paridevana' means lamentation, wailing, the kind of grief that makes a person beat their chest. Krishna's question is almost gentle: once you see this pattern clearly, what remains to wail about?