I remember my first time reading through the history of 1800's America: And seeing the constantly, constantly, constantly evolving US flag...as state after state got added, and the stars had to be reconfigured in all sorts of ways. But even as a schoolkid, I thought those days of an expanding union were behind us. That, after Hawaii's addition in 1959, that was that.
So maybe it's the schoolkid in me, getting excited at this. A bit of youthful idealism in the idea of America. But I can't help but be excited at the possibility of Puerto Rico as the 51st state.