Answer
as someone else said on here
"explained its upward movement towards the sky as eternal. The poet says that, similar to the rain, a song starts from the heart of the singer, travels to reach others and, after fulfilling it purpose (whether cared for or not) returns to the singer with all due love. "
i think of it as the sky is the limit , so like it can go on forever as far as we know. There isn't an end.