I don't know what it is but I will very rarely stop reading a book before I've reached the end. Even if halfway I'm bored to death I will keep at it, hoping that somehow on the next page the story will pick up, the characters come alive, ...
What do you do: finish what you've started or pick up another book?
The only book I can recall I've stopped reading after about a hundred pages or so was Peter Carey's Illywhacker (which I'd started because I liked Jack Maggs).