in library books

I always find it interesting which passages and selections have been highlighted, starred, and underlined in library books. What one person finds relevant or interesting might be something that I would have skipped right over. I stop and take notice though, because it was marked. This makes me stop and rethink, does it have meaning? Am I missing something in this passage that I should be getting? Different people read for different reasons, and this probably explains the differences, but it still makes me stop and think.