Some interesting reasons to stop...
The post "Why Do You Abandon A Book?" by Katie Lepi features an interesting infographic compiled from a GoodReads survey. While the titles are adult and not really usable for school, I like the idea of surveying your own students and finding out a) what makes them stop reading, b) how long will they read before throwing in the towel and c) what might keep them reading even if the book isn't a grabber? (You could even create your own infographic!)
We probably need to empower our students more in their reading. This always makes me think of Pennac's "Readers' Bill of Rights".