There are so, SO many great books out there, and only one hundred years you will live. At most, what, about 1,000-2,000 books you can finish in your lifetime (and that is incredibly generous)? Why waste time rereading a book if you can read a brand new - maybe even better - book?
I spent more than a year, maybe two, reading/rereading the Harry Potter series. (Seriously.) I barely read anything else other than Harry Potter; once I finished one book in the series, I'd go on to another (although, I've never read them all one-by-one in order. I should do that sometime....) I think I read Deathly Hallows fourteen times (give or take a few).
I feel like, maybe, I regret that - I could have read so much more, but whenever I had tried, I usually put the book down cursing that it was nowhere near as good as Harry Potter.
I understand rereading books - I've done it a LOT. I understand how it can be bad. But I understand how it can be amazing too.
I recently started rereading The Great Gatsby and I found out JUST HOW MUCH I MISSED THE FIRST TIME AROUND. I skimmed through the first half or so the first time I read it, and this time, I took my time, and found so much more. (Maybe my problem is speed-reading, which leads me to rereading.) I picked up on so much foreshadowing and hints that one could only realize the second time around.
Some books become totally different the second time they are read. Harry Potter, I feel, is one of them; it is a true regret if you do not reread those books. The plot is magical and should be read/studied by every writer.
Usually, books with mysteries and plot twists fall under the 'rereadability' category, yet, I might reread Catcher in the Rye just because I love the writing so much. And that book almost lacks a plot.
Rereading is great if you have forgotten a book's substance but still remember loving it. REREAD IF YOU ARE IN THIS SITUATION! It'll be (I think) an amazing experience :D
So is there a 'criteria' to reread?
In the end, it comes down (as it always does) to balance.
I wouldn't recommend doing what I did and only reading Harry Potter for a couple of years. That was bad - but I wouldn't say unnecessary, exactly. Maybe it was just something I needed to do. I haven't reread any Harry Potter book since Deathly Hallows, back in 2011 when the 8th movie came out. (I finished the reread in the IMAX theatre with (literally) 50 other friends, during the birthday/movie party which I held. BEST BIRTHDAY EVER (so far)!)
Basically, if you feel like rereading a book, go for it. If you don't feel like it, don't. Keep it simple and life will be easier for everyone.
I don't like the idea of rushing to read every book out there before you die - why not enjoy what you are reading? If at the end of your life you regret not rereading your favorite novel, then maybe your super-fast reading-spree was harmful.
Be okay with the fact that there will be great books you will never read - either those already out, or those out after you are gone. Enjoy reading. Doesn't matter if that means reading something new, or rereading your old favorites. Live happy :)
How about you guys? Do you like to reread, or would you rather read new books every time?