Many say that they would love to write a book, they have the idea, and the desire, but for most people they never get the book written. It could be many reasons. You may scribble a few lines and lose focus. This is a discipline. Another reason is self doubt.
Many writers have a problem with insecurity.
When you write a book you are baring your soul to the world. Many of the best books out today have been rejected many times. If you believe in your story you will keep on perfecting it, and keep on sending it out, and you will stand by it when it is published. Another reason is isolation from other writers. If you have never met another writer before, you may be holding them up on a pedestal that doesn't exist.