You're a writer. You've spent months, even years, slaving away in solitude, attempting to transform a great idea into a Great American Novel. And you've succeeded.
Critics love it.
Readers love it. Hollywood luuuvs it and proves just how much by forking over megabucks for the screen rights. The next thing you know, filmmakers are changing the life story of your leading lady, adding scenes and characters you never dreamed of, reinventing your ending, and generally performing so much plastic surgery, you no longer recognize your own baby. Films and books are very different mediums. A movie must tell its story through images and action, while books are all about words.