I find it impossible to imagine that you can draw all the value out of any serious work of art in one pass. I read a book once just to have read it, just to see if it's worth my time, really. Then I'll read it a few more times so that the details are clear in my head -- so that I know the plot, the character arcs, the biggest style factors, the organization and can start formulating my ideas about the book. After that, if my ideas warrant, I'll re-read any number of times, often looking for specific details to support or challenge a particular theory about the book.
And rereading my favourite stories is like visiting old friends. I mean, you could travel to a new neighborhood everyday, couldn't you? But instead you come home. To the people you know and love and trust. Even if you end up having the same conversations over and over again, sometimes it's awfully nice to go home
I like visitng with Fafhrd and Mouser, or with Vlad and Kragar and Morrolan and Sethra, or with John Carter and Dejah Thoris, or with Croaker and One-Eye and Lady and Limper. Reading their stories is a way of spending time with these people who have come to mean something to me. A way of going back to a neighborhood full of happy memories.
Two reasons for re-reading: to examine a work more carefully, or to put a smile on your face.