The poem opens with the speaker declaring, "i carry your heart with me(i carry it in my heart)." He goes on to stress the sense of unity he (we're just assuming it's a he) feels with the one he loves. Everything he does and feels is connected to her (again, just our gender assumption here). His fate and his world is her alone. The meaning of nature is also shared with the speaker's love. Finally, the "tree of life" or the "secret nobody knows" has its roots in the wonder of love and its limitless possibilities. All together now: aww.