I'm not really a big fan of Stephen King. I think some of his books are longer than necessary and he needs better editors. I do like some of his short stories though, and I believe the truth of why his writing is so compelling is because of his understanding of human psychology.
I was reading this essay on Mental Floss earlier about the story behind King's first bestseller, Carrie. Stephen King started his writing career sending his stories to men's magazines for tiny paychecks (if he's lucky). Someone accused him of not being able to write a female character convincingly. He decided to pick up the challenge, and the idea for Carrie emerged from his brain.
King modeled Carrie White after two of the loneliest girls he remembered from high school. One was a timid epileptic with a voice that always gurgled with phlegm. Her fundamentalist mother kept a life-size crucifix in the living room, and it was clear to King that the thought of it followed her down the halls. The second girl was a loner. She wore the same outfit every day, which drew cruel taunts. By the time King wrote Carrie, both of those girls were dead. The first died alone after a seizure. The second suffered from postpartum depression and, one day, aimed a rifle at her stomach and pulled the trigger. “Very rarely in my career have I explored more distasteful territory,” King wrote, reflecting on how both of them were treated.
It made me think about Stephen King's writings - the ones I have actually read: The Shawshank Redemption, Different Seasons and The Green Mile - his "non-horror" works. King as a writer, is an astute observer of human nature. But more than that, there is deep compassion in his stories.