Do it all the time. In fact, I should think that 60% of my reading consists of re-reads. For a start, it saves me a fortune.
Perhaps I've never grown up properly.

Kids love to have a favourite story retold time and again, and so do I. I think our enjoyment comes in the telling, not simply the ending.
Do you not think that a similar attitude applies to films & TV series? How many of you who never re-read a book also never watch repeats of films? I'm not making a point here; just asking out of interest. I can re-read a book or watch a repeat of, say, Morse, much more frequently than I can watch a repeat of a film (I know that doesn't seem to make sense, but it's true).
My mother would occasionally start to read a library book with much enjoyment then at page 100 or so suddenly realise she'd read it before. At that point she'd stop reading it. I never quite understood that.