Why does Rex reject the practical gifts Jeannette gives him for Christmas?

Rex is not a practical person. He lives on grand ideas and often delusions of grandeur. Consider what he gives Jeannette for Christmas. Rex offers Jeanette a star in the sky for Christmas but then she picks out a planet instead. She picks Venus and her dad says that's her Christmas gift from him. It sounds sweet but it is also representing the fact that Rex cannot accept or give anything substantial.