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Answer by Marcus Geduld:
Books are machines built specifically to excite your emotions. (Not all books, but almost all fiction and a large amount of nonfiction.) Authors spend their lives studying how to evoke and manipulate emotion and pour all that knowledge into their works. They even do it on the sentence level. Writing teachers urge their students to include some sort of hook in each sentence that will lure readers to the next one.
One common technique is ending chapters with cliffhangers, which excites our instinctual need to know what happens next—instinctual, because learning results was crucial for our ancestors' survival (e.g. when a tribe member ventured into a cave, his friends needed to learn if he emerged safely or got eaten by a tiger inside). Other techniques include putting likable characters in peril, which evokes empathy (another evolved trait), and lacing stories with surprise, which tickles our natural pattern-matching instincts by thwarting our conclusions.
There is also a natural process that changes the brain over time as we get to know a person. It's a large part of the engine behind friendship. If you just meet me once at a party, you'll probably forget me afterward or remember just a couple of funny, interesting, or strange things I said. But if you spend lots of time with me, you'll grow brain structures devoted to simulating me and predicting my behavior.
This happens because we evolved as pack animals—as tribal creatures. Huge portions of our brains are devoted to social processing. There's even evidence to suggest we unconsciously think about other people when our minds seem to be at rest, zoned out, and not thinking about anything. Back in our tribal days, who was sleeping with whom, who was in charge, who was likely to betray you, and who would most likely come to your aid were crucial bits of survival information.
We rely on those social brain structures to make judgments like, "Don't talk to Sally about her mom. It really upsets her," and when we're picking a birthday present for Jonathan. You have a little sim of him in your brain. You mentally give the sim various presents, see which ones most delight it, and that's how you decide what to get for the real-life Jonathan. It's why you feel confident saying, "I knew you'd like this!"
Metaphorically, at least, you can say that someone you've grown used to has inhabited your brain. You hold a copy of him in your head—not an exact copy, but one that's good enough to excite your intellect and emotions. This is why it's so hard to let go of a lover who dumps you or a friend who dies. He's gone but not gone. Your brain is still running his program.
Books, especially long character novels and biographies, can have the same effect. Read Lord of the Rings and you wind up hanging out with Frodo for a long time, long enough for your brain to start simulating him, even when you close the book. You can also create sims by rereading the same book over and over, throughout your life. Lots of people have complex simulations of Harry Potter or Jay Gatsby running in their heads. Once a sim starts running, it's bound to affect you.
You will find yourself wondering what happens to fictional characters after the book is over. On a purely rational level, that makes no sense. After the last page of Nicholas Nickleby, there is no Nicholas Nickleby anymore. There's nothing that happens to him after the book is over. Yet there is, because there's a Nicholas Nickleby program still running in your brain, still predicting what he'll do, still exciting your empathy.
Books offer condensed social information. They are to our tribal and social concerns what candy is to the parts of our brains that crave fruit. We're obsessed by anything social and causal, and books cater to those obsessions.
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