I've had rereading on my mind since Nicole posted some thoughts on the subject. Even though there are more books on my 'to be read' pile than I can hope to get through, sometimes I'll read a book anyway. I've tried to figure out why its sometimes worth the effort, and came up with a few reasons:
1) Different state of mind. I've never been able to enjoy any book I've read while traveling. I think it's because airports stress me out, and there's too much stuff going on that I can't focus. I've reread books I started while I was away, and really enjoyed them the second go around. I've also reread books I had good memories of when I was younger, and had a completely different reaction now that time's passed.
2) Comfort reading. Sometimes I'm an emotional wimp. If I've read a book before, I know what's going to happen, so I know how well I can deal with the subject matter. When I'm feeling a bit romantic, or lonely, or stressed out, and know there's a particular book that will satisfy some sort of requirement, I will reread it.
3) To learn. I've come back to books to see how a writer handled craft issues that I'm having difficulty with. My books these days all have little torn pieces of paper in them that I mark as I read. I've also made a habit of noting down what things every writer does well, so I can look back at how they handled it. This might not require a total reread, but skimming different parts at a time.
Do you reread books? When do you re-read a book? Are they your favorite books? Or most challenging books? Or maybe you only read certain passages over again? I'm curious!