Because when you read a book, you feel engrossed in it. Reading involves our imagination of ourselves as the character. If you feel that you're the character, it means that book has you hooked up.
Soon, you start to think how creative the writer of that book is. You start to think how awesome that writer is because you feel the book is great. And then , you feel like writing your own or if you've already started and took a break from writing, this thought would pull you back to your own writing. And you feel restless until you write down some pages. Even if it is not more, it something.
That is all you need to complete your book, because once you start you won't stop until your story is done.