At the time this speech was given, Berlin was a divided city. And Kennedy's language plays on that throughout—what a clever dude that JFK was.

Beginning and ending with his assertion that he too is a citizen of Berlin, Kennedy makes the case that Berlin can't be divided because we are all one. The entire speech feels like he's about the ask everyone to hold hands and sing together—in a compelling, diplomatic way, of course. He speaks almost as if he's talking to a single person and not a crowd of hundreds of thousands. One giant Berliner joined together by love.

This motif culminates in sentence 23:

Freedom is indivisible, and when one man is enslaved, all are not free.

This is one of many assertions of togetherness. Forget the wall, we're all one in a cosmic sense. His indivisible idea is that we don't have live on the same side of the wall, or even the same side of the ocean, to be together.