1. Because it's dense and complex and you can get even more from it the second time around.
2. Because it's fun to go back and see the forshadowing for what it is, or to try and spot the clues.
3. Because sometimes you need a little comfort food, or a visit with old friends, and a favorite book is like the best kind of visit home.
4. Because there's nothing new you want to read (hahaha! Does this actually happen to anyone? Because there's always something new -- well, new to me anyway -- that I want to read).
5. Because you need something to read that you can easily pick up and put down because you're reading in snippets in between doing something active yet inactive like nursing -- because you just had a baby.
Number 5 is my current reason. :-)
Are you a re-reader? Do you pick up familiar books for any of the above reasons, or for something entirely different?