In my opinion, this poem is sort of a mirror image of the Pandora myth. In other words, it has God doing the exact opposite of what Pandora did.
In the Pandora myth, Pandora was curious and she opened the box or jar or whatever and let out all the bad stuff. At the end, all that was left in the box was hope.
By contrast, in this poem, God has a glass full of blessings. He takes all the blessings out of the glass and spreads them over the world to help make us happy. The only thing he leaves in the jar is rest.