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Because the paper fibers are quite flexible, but not very stretchy, they're not easy to compress; when you try to do so they're more likely to bend and then re-straighten. However, when you roll the piece of paper up the fibers on the outside of the paper, because they don't stretch very much, get pulled slightly apart. When you then release the piece of paper, the fibers that were pulled apart don't suddenly jump back to their original position; that would take even more energy, so they stay in their new position, which means that the outside surface of the sheet of paper is slightly greater than the inner surface, so it stays curled....I think.
It's the same reason that there has never been an assassinated Australian politician: it's basically civility.cheers
When you get a thin piece of paper, roll that up, un-roll it, it doesn't have that same 'Rolled-up-ness' as a thicker one.