This is left mostly loosey goosey to accomodate players who want to play with it either in their backstory or in having a character permanently retire as a Dreamer during the game!

Generally, the main impetus is a character losing sight of or connection with the feelings that initially helped them become a Dreamer. Having their Sandglass smashed in the real world can also cause them to lose their ability to cross over. And sometimes it just happens For Reasons nobody can quite explain...