i carry your heart with me(i carry it in my heart)i am never without it (1-2)
From the very beginning, we notice the way Cummings is using language and punctuation to further his idea of unity and love. The parentheses and adjacent words are squished together to suggest the inseparable nature of the speaker's relationship with his lover.
i fear no fate(for you are my fate,my sweet) (5-6)
By the second stanza, we hear more of the idea of love being part of a larger picture. The speaker fears no fate because his lover is the world to him. So, no matter what happens to everything else, his fate can never be feared so long as he has his "sweet."
here is the deepest secret nobody knows (here is the root of the root and the bud of the bud (10-11)
Love is the "root of the root," so it's therefore the foundation for everything else. And it's the "deepest secret nobody knows" because it's so darn mysterious and yet so essential to our livelihood.