There are only two kinds of people: those who say to God, ‘Thy will be done,’ and those to whom God says, ‘All right, then, have it your way

Lewis' story intertwines what the main character is experiencing with what everyday people experience. In The Great Divorce, the people in Hell are stuck there by choice. These people choose to live in a dull world, where everyday they grow farther apart from the other inhabitants. They have the opportunity to leave on the bus, but that has its own price. 

The dead arrive in Heaven on the bus, but Heaven is painful for them. They have to grow accustomed to the good things in Heaven. To do this, they have to let go of what the have held on to in life. This proves difficult for most. This ranges from anger at a friend to a man who always thinks he knows it all.

This is a metaphor for the lives of humans. Most people would rather live in a dull world where they think they get do do their own thing. However, all that they really do is grow apart from people. Then, when they are confronted with Christ, it hurts because there are things that they refuse to let go of. 

And when they finally let go, they become strong and can help others let go of their demons and find God.