There are some films I'll watch over and over again - 13 Going 30, You've Got Mail, Bridget Jones' Diary and Picture Perfect to name a few - and I'm the same when it comes to books.
If I've loved a book so much it's like losing a friend (or a whole bunch) when the last page is turned, why not pick it up again and rediscover the story and the characters all over again? I find re-reading books comforting. I already know the characters and I know what the outcome is going to be, so I can really settle in and simply enjoy the journey. I have a handful of books I've enjoyed time and time again and never tire of re-treading the journey with the characters:
Kiss Him Goodbye (Victoria Routledge)
These books have been with me for a long time as I first read them during the early 2000s. They're a bit battered after so many re-reads but I love them as much now as the first time I opened them and they have pride of place on the top shelf of my bookcase.
Sometimes a book has a follow-up and, just like I'm rewatching the first series of ITV's The Bay at the moment as a refresher for series 2, I'll sometimes need a re-read of the original, especially if it's been a while between books. This is the case with Dorothy Koomson's All My Lies Are True, which is a follow-up of The Ice Cream Girls. I read (and loved) The Ice Cream Girls around 2012, so I'm definitely going to need a refresher re-read before I dive into the next book!
And then there are the classics you've been reading since childhood. In my case, it's The Animals of Farthing Wood, which I've been re-reading since 1993. My copy is so battered and fragile after being read and carried around in backpacks and handbags over the years that I've bought myself a new copy. I still love my original, but it's having a rest on my shelf so it won't fall to pieces during its next outing.