Sometimes I re-read because I want the comfort of that story. Also, I tend to read fast so re-reading is sometimes finding things I missed the first time. Subtleties I missed in my hurry
Sometimes because it's been years and you can't quite remember it anymore but you remember you liked it.
I suppose, with all the books in the world, why would one re-read a familiar story? I think it's inherently human. Look at kids, they lurve to watch the same video or hear the same story over and over and over and over again.