What makes a classic?

I have been wondering, what makes a classic book? What elements of a story makes you read and reread one book?

For me, Lord of the Rings and Pride and Prejudice are two books that I have read multiple times. Both are very different, one is an epic fantasy with fantastic creatures and magical lands and the other is a book on social commentary set in an earlier period with dances and focus on status, manners and decorum. One has epic wars between orcs, elves, dwarves, humans and the other has an epic showdown of words between passionate characters (Darcy and Elizabeth). Humanity centers in both of these books.

So what makes a classic read for you?