here is the deepest secret nobody knows (here is the root of the root and the bud of the bud
The third stanza starts with some anaphora in that repetition of "here is the." So things are sounding very lyrical without all the perfect couplets the romantics might have preferred.
But where is "here"? If we go by what we saw in the previous stanza, we're guessing the "here" refers to the foundation of the speaker's love, its true essence, which everything revolves around. The "here" might even be referring to the poem's refrain and the idea of the speaker always carrying his lover's heart. That heart he carries is the "root of the root," the bee's knees, so to speak.
And what's all this about a "deepest secret nobody knows"? We can assume the speaker is driving at the deepest essence of love, since love is what the poem's all about. And nobody knows the secret because love is kind of the greatest mystery of all.
But we also know it's probably more important than anything else.
We've also got some more ambiguity with all the secrets, "roots," and "buds." By now we understand that specifics don't matter to this poem because the only thing that's important is the speaker's unity with his lover and their love. Everything else is defined via their love.
So line 11 makes even more sense to us when we imagine "roots" and "buds" that are the symbolic foundations of life. You can't have trees without roots and you can't have pretty flowers without buds. And since the speaker says it's the "root of the root" and the "bud of the bud," we understand that love is therefore the deepest foundation of all, since it goes beyond the roots and buds.
and the sky of the sky of a tree called life;which grows higher than soul can hope or mind can hide)
So we were just in the ground with the roots and now we're in the sky, where trees exist too. It looks like the speaker is blending it all together to show that love is the unifying force that keeps the roots, buds, and the sky together in this "tree called life."
We're also noticing some nature imagery and metaphors that the Romantics would likely get very excited about. So the speaker seems to be giving more of a nod here to Blake and Co.
Notice too the repetition of that "of the" clause. The more the speaker uses that clause, the deeper and bigger we seem to get in terms of love being the ultimate unifying foundation. So again, Cummings is demonstrating his use of syntax to not only keep things sounding lyrical but also to be as efficient and precise with his language as possible.
By line 13 we understand that the "tree called life" can figuratively grow to limitless heights because it's founded on (have you guessed it?) love!
Even those pesky "souls" and "minds" can't fully grasp the full extent of life without love. It'd kind of be like having a Big Mac without the patty. (Romantic, we know.)
Notice how the speaker includes the idea of the mind never "hiding" the full potential of life and love. Here he's saying that reason can't outperform love. Likewise, love can't be rationalized or "hidden" in theoretical hoopla.
Finally, this parenthetical clause is a lengthy one with three lines. And it kind of makes sense that it's so long since these lines delve more into the general idea of love. The speaker's not just focused on his lover at this point; he's thinking about the bigger picture of love.
and this is the wonder that's keeping the stars apart
Just in case you weren't sure if love is the greatest mystery of all, line 14 sums it up by letting us know that it keeps "the stars apart."
So there's no need to search for meaning and purpose in love because in this poem it's kind of the defining element of the cosmos. It just "is," so that should be enough.
To have love is therefore what this "tree of life" is all about. Since its roots rest in love and its potential is limitless, we understand that its "wonder" sustains everything else.
We also get by the end of the third stanza that the speaker isn't just talking about the romantic love he shares with his partner.
We're seeing "love" in a more general sense here that's linked with the cosmos.
So sure, love may be difficult to qualify and rationalize, but that's kind of the point. If love were easy to explain (and write a poem about) we wouldn't have that "wonder" the speaker is referring to.