Often times, the reader simply wants to see a happy ending to the story. For example, if the main character is involved in a love story, they want to see the couple married, living in the perfect house, and starting a family. If it is a mystery, the reader wants to see it solved and the character living happily. As long as there is a finality to the story (possibly a death or a fresh start at a new life), the reader doesn't need the story to go on.
