What keeps you glued to a book that you don't like?

I've noticed it several times in myself, at least. The most recent example being, as embarred as I am to say it, the "Twilight" book. My sister brought it home, and since it was all of the TV with the movie, and what not, I thought I'd check it out. It is painfully redundant and derivative. But I just kept reading. I kept saying to myself "Christ Almighty this book is awful!" but I finished it in less than a day, none the less. Why? Why do I keep reading a book I don't like? Any idea? It happened with the Da Vinci Code, and many others.