I think interest in the book's subject matter is one of the important factors.
Another could be the reader's relationship with the author. I can't thank enough family members and friends who have supported me with my books.
Finally, if your friend or loved one is reading a particular book, and you want to share in the experience of that person - perhaps to have something to talk about, or even to help junior with the homework he's been struggling with - then that can also encourage you to read whatever book the other person is reading.
I'm sure there are more, but these would probably be my top three reasons. Thanks for the opportunity to answer this question.