It's no secret kids like repetition, especially when it comes to books. When I was little, I always checked out either A Boy, a Dog and a Frog, New Blue Shoes or Harold and the Purple Crayon from the Bookmobile when it stopped at the school at the end of our street. When E and Z were about two, I hid Trouble With Trolls under the bed because I got sick of reading it after about fifty nights in a row.
But this desire to hear the same words over and over again doesn't stop with picture books. C gave Peter Pan and Wendy to M for his fourth birthday (the birthday that fell three days after his brothers were born), and over the next year read that hefty tome to him at least four times. Once he started to read on his own, he read the same books over and over again (I think he's read the Percy Jackson series about ten times).
One of the first chapter books I read to E and Z (after Winnie the Pooh, of course, which I'd happily read again), was Trumpet of the Swan. As soon as we got to the end, they both insisted, "Read it again!" I enjoyed the book, sure, but I didn't want to read it again right away, so I managed to divert their attention to other books.
About two years ago, C and I started reading them the Harry Potter series. It took most of a year to get through all seven books (and I ended up doing most of the reading, because C got interested and read ahead on his own). As soon as Harry put Voldmort to rest in book seven, the familiar refrain of "Read it again!" rang out through our house. By then, I'd had enough of stumbling over words like "Wizengamot," so I refused, instead reading them books like The Secret Garden. C, however, began all over again, reading them The Sorcerer's Stone. They only made it partway through book four, however, when their attention got drawn away by other books, like the Secret Series.
A couple of weeks ago, after we finished Farmer Boy (to which the boys put up great resistance initially, but then got sucked right in), E begged to hear Harry Potter again. It had been long enough since our first go-round that I didn't mind delving into it again. It's been a good opportunity to teach them about foreshadowing––Hagrid borrows Sirius Black's flying motorcycle, "Foreshadowing!" And it has proved kind of fun to read again, and comfortable, knowing what is going to happen to all of the characters in the end. I think I see why kids like the repetition.