Have you ever laughed until you've cried, or cried so much that you just had to laugh? People aren't equipped to feel one emotion all the time, and that's why they can't read books that are just one thing. Whether you're writing a comedy or a tragedy, all good books have elements of both. That's what makes them good...because life is both.
Walking the Line
By a certain school of thought, all books fall into one of two categories: comedy or tragedy. But within that framework there are infinite story possibilities, numerous twists and turns. No story should ever be all comedy or all tragedy, because every reader will reach a point when they can't laugh or cry anymore. But if you learn how to jerk them back and forth between the two, they won't be able to stop reading.