What do you think Postman means when he says there's nothing wrong with building castles in the air, but that, "The problems come when we try to live in them"?

Could someone me with this question?

Aman lives on the twelfth floor of an apartment building. every morning he takes the elevator down to the lobby and leaves the building. in the evening, he gets into the elevator, and, if there is someone else in the elevator -- or if it was raining that day -- he goes back to his floor directly. otherwise, he goes to the tenth floor and walks up two flights of stairs to his apartment.

What is the origin of déjà vu, and what is its meaning?