I'd never really thought of it like this, but Joyce Carol Oates nails it:
Reading is the sole means by which we slip, involuntarily, often helplessly, into another’s skin, another’s voice, another’s soul.
I came across this quote here, where it kicks off a very interesting meditation - by a psychologist, not a novelist - on how empathy works in fiction. One of the most interesting things it mentions is that people who don't empathise easily in real life have a stronger empathic response to characters in a movie if they think they're fictional, than if they think they're real. The suggestion is that empathy in real life is dangerous for people like this (and we probably all are like this to some degree, sometimes): it involves relaxing your guard, becoming vulnerable to someone who may take advantage, or whose pain may destroy us, whereas empathising with fictional characters will do no worse than, say, make you cry at the end of the novel.
This makes it sound as if reading is living lite, as it were: reality safely eviscerated. And, certainly, most of us read for anaesthesia on occasions, with books that either are so familiar they're like singing a nursery rhyme while you do some dull job, or which present a small, familiar, safe and ultimately comfortable experience of the world. But anyone who's been haunted by a book for years, or read madly, terrified, deep into the night, or felt their life or how they see the world rearranged by a story, knows that a book is a lot more than lite. If we can slip further into another life and soul through fiction than by any other means, then we can also be more open to whatever joys or horrors live there. (Which gives rise to a tangential thought about how and whether writers should try to make their main character likeable, or even, depending on genre, lovable. But that's another blog post.)
And I'm reminded of a point which was made in Richard Kearney's book On Stories. When Spielberg made Schindler's List, with enormous care for historical accuracy and respect for the truth, it was attacked by the director of Shoah on the grounds (among others) that making a story out of the testimonies of Holocaust survivors - a piece constructed to fulfil the needs of narrative form - was, inherently, unethical: that it betrayed the nature of those experiences, since they were broken, beyond story-making or rationalising into cause and effect or narrative arc. The only way to transmit them, goes this view, was as Shoah does, in direct statements by survivors to the camera. But Kearney quotes some survivors who were unable to watch Shoah at all, who had never revisited their memories, but who could watch Schindler's List. He also mentions that some of the speakers in Shoah, bring out their memories for the first time, found them unbearable to live with and later committed suicide. In other words, for some the prism of enactment, mimesis, re-creation, is the only way they can safely approach such memories.
Many English teachers (and slushpile readers) would say their heart sinks when some wild or peculiar story ends 'and then he woke up.' They discourage it because it seems to be cheating, as some readers feel speculative fiction is cheating, because you can do anything, however incoherent, without anchoring it convincingy, and then have a get-out clause. It was one of my children's teachers who pointed out that often the child, having gone deep into its imagination and found some strange, scary, perhaps threatening things there, may need to use the story itself to bring them back to safe, boring normality.
It's not just their own stories, either. Even if you don't buy into the full Freudian package, Bruno Bettelheim's classic The Uses of Enchantment is still utterly fascinating in how it explores the deep psychological events and states which the classic fairy tales embody, and embody in a form which is not too real, not too close to home, but which can nonetheless work through the fears and angers which we must resolve in order to grow up. Not that they ever go away. Sure, Max sails home and finds his supper still hot. But the wild things are still out there, and it's in reading that we can go and find them again, when we need to.