and it's you are whatever a moon has always meant and whatever a sun will always sing is you (8-9)
These lines could have very easily become a romantic cliché with the moon and the sun. But Cummings makes them ultramodern by questioning what a moon is supposed to "mean." So the speaker is amazed by his love but he's still a bit real and modern.
and the sky of the sky of a tree called life;which grows higher than soul can hope or mind can hide) (12-13)
As the speaker's amazement of his love grows, so does this "tree called life." In fact, he's so amazed that not even his "soul" or "mind" can fully grasp the extent of how much his love has affected him. So the world is looking pretty cosmic to us at this point.
and this is the wonder that's keeping the stars apart (14)
It wouldn't be an awe-inspired poem without a little "wonder," right? And the way the speaker phrases this wonder that's associated with the stars is also ultramodern. Notice that he's not talking about stars in his lover's eyes. Instead he imagines how love "keep[s] the stars apart." So we've got yet another modern, but still awesome, way of looking at the world.