I studied literature in college, and throughout my twenties I voraciously read contemporary fiction. Then, eight or nine years ago, I found myself getting — well — bored.
Why? I think it's because I was reading novel after novel about the real world. And there are, at the risk of sounding superweird, only so many ways to describe reality. After I'd read my 189th novel about someone living in a city, working in a basically realistic job and having a realistic relationship and a realistically fraught family, I was like, "OK. Cool. I see how today's world works." I also started to feel like I'd been reading the same book over and over again.
Here's my overly reductive, incredibly nerdy way of thinking about the novel: Consider it a simulation, kind of like The Sims. If you run a realistic simulation enough times — writing tens of thousands of novels about contemporary life — eventually you're going to explore almost every outcome. So what do you do then?
You change the physics in the sim. Alter reality — and see what new results you get. Which is precisely what sci-fi does. Its authors rewrite one or two basic rules about society and then examine how humanity responds — so we can learn more about ourselves. How would love change if we lived to be 500? If you could travel back in time and revise decisions, would you? What if you could confront, talk to, or kill God?
Teenagers love to ponder such massive, brain-shaking concepts, which is precisely why they devour novels like Philip Pullman's His Dark Materials, the Narnia series, the Harry Potter books, and Ender's Game. They know that big-idea novels are more likely to have an embossed foil dragon on the cover than a Booker Prize badge.

Clive Thompson. "Take the Red Book: Why sci-fi is the last bastion of philosophical writing."