The reason why the sky is high is for people to look up at, just like the stars that are out of reach can only be seen from a distance. The city in the sky in everyone's mind is the unreachable stars, but everyone believes that they can reach that high city.
God couldn't bear those humble human beings to decline one after another, so he discussed with Satan.
Satan said: "Come on, there is everything you want here, as long as your efforts satisfy me."
One day, an angel who knew nothing about the world passed by and asked the guard guarding the door, "Where is this place?" "The back door of hell!" The guard replied expressionlessly.
The angel remembered what God said: Human beings, you think you choose heaven, maybe it is hell.