Hey Ian,
Let me try to tell you what "re-reading" really is to me. When I read a book that I'm extremely fond of, I take notes --subconsciously or not-- of the scenes in it that attracted me the most, and most of the time, that's what I remember the book by.
Some things trigger the memory of one of those scenes and make me think about the story --whether it's a movie, something someone says to me or whatever... and when I have time and the books at my disposal, I like to re-read that particular scene and be lost again.
Sometimes I finish the story, sometimes I begin from the start, sometimes that one part of the book satisfies my needs.
That's not always how it goes though. Sometimes I just get stressed or feel bouncy or just don't know what to do with myself, and I need something to calm me down, something familiar, something "soothing" if you like; something to hold on to, to keep me from getting lost inside my own head. The fact that I know what's going to happen in the story (I know some parts of the books I know by heart, don't ask me why) helps me get through those moods.
Re-reading books, re-feeling the same emotions I felt the first time, together with new emotions, keep it from getting boring. I have to admit that I really like to re-read books I read in my childhood as well. Makes me feel comfortable, makes me feel at ease. I know I must've read one particular book about 50 times over the past years. Crazy? I don't know, it's just who I am I guess.
Reading, to me, is much like listening to music. I don't get bored of songs, I don't get bored of books. If I hear a song and like it, I know that feeling is never going to go away. Same with books. There's always something new to discover in the process of reading or listening.
Maybe the fact that I'm not really a "thriller-girl" has something to do with it as well... I like my books to have a nice flow to them: just waves of words to crash upon me. This sometimes leaves out the "twist in the tail" part, though that sort of thing does occur in other genres as well, I'm aware of that, but that doesn't bother me none.
I hope this clears things a bit up? I really don't know how to put it differently. It's just a feeling I have about it, and I'm not that good at explaining feelings...