In true Romantic poetry style, the speaker ends by (almost) perfectly referencing the title. Notice the omission of "with me"?
So line 15 sounds a bit tongue-in-cheek, since he's keeping with a normal poetry convention (but not entirely). You probably saw that coming, right?
After all the cosmic, "tree of life" stuff, we're back to where we started, with the most important idea of the poem: the speaker carries his love within him at all times.
And we're also reminded of the unity that he feels with his lover with that squishing of words and parentheses.
In true modern poetry style, this stanza only has one line. So Cummings is making it clear to us that line 15 is the point of the whole poem. It even gets its own stanza because it's so important and special. (Check out this one-line approach here too.)
Our speaker has traveled the globe of love in only 15 lines, but he reminds us here that the love one personally experiences is most important. To quote another poet: "All you need is love."