As we near the end of "if everything happens that can't be done", we discover that our speaker is in love, and suddenly we understand why this poem has been so full of joy. He loves, and is loved, and in that love feels that nothing is impossible, and that the world is wonderful. You just might want to keep this poem in your back pocket come next Valentine's Day.
This is not a love poem. Instead, it's a poem about knowledge, and where that knowledge is best found.
Everything is happening that can't be done because the speaker of this poem is in love. He's on Cloud Nine.