The book is a light intro into the field of Self-Determination Theory and it's filled with examples of people following their passion for 6 months before flaming out because no one will pay them for the thing they like to do.
Another way to put it is that making money takes practice just like playing the piano does. You're probably not going to wake up one day and realize that an empty calendar is exactly what you need to learn how people will pay you for something that nobody told you to build.
http://37signals.com/svn/posts/1985-making-money-takes-pract...
http://www.inc.com/magazine/20110301/making-money-small-busi...