For me, the principal reason for returning to the Moon, is because of the Overview Effect.

It is an experience triggered by seeing the Earth (as a whole) from space and realizing the Earth's inherent unity and interconnectedness. Globalization is one of its manifestations. We live in a planetary civilization. I'm constantly in contact with others around the world.
From the Moon, the Earth is nothing like seen from low earth orbit. From the Moon, the Earth looks like a tiny oasis for mankind in an enormous and unknown universe. From there, the Earth doesn't appear in a clear blue sky or with clouds or with thousands of stars...It appears very small, very beautiful (the kind of beauty we call so because of our culture and genes) and extremely isolated in the pitch black sky.
With the overview effect comes the realization we are as much part of the universe as we are of earth.
Ever more so if humans are on the Moon and we on Earth know this and this will bring the realization we are part of the Universe, the Earth is a spaceship traveling in it, moving around itself and inside the solar system...and we need to take care of it.