First, typing in words what you see in your head is torture. You throw every word out for places, people, and situations only to look back and discover that the words aren't enough.
Second, filling in the details requires embracing the human experience. I don't care if it's a Sith Warlord fighting his son, a teenager that loves a vampire, a guy watching the life of his millionaire neighbor, a man dying at the base of Kilimanjaro, or a kid surviving magic school; they all have to appeal to our humanity. Capturing the essence of human emotions is difficult for an introvert that is emotionally retarded.
Third, filling in the details can require visiting some dark places. My emotional retardation is more to do with expression of emotions rather than having them. Writing requires shaking the bottle where I hide them and pouring out the mix.
All of this is what Ernest Hemingway meant when he said the following:
So if I hate writing, and I hate sharing my emotions, why did I write a book? It's that intense feeling after a good writing session that drives me. I've never been in "The Zone" before. The zone that athletes, artists, musicians, actors, and some business-people enter when they preform. I've had times when I'd look at an activity and think, hey, this is fun. But never before seriously writing my book did I ever experience the flow state of "The Zone." After these writing sessions I'd look up at the screen, fingers aching, head throbbing, hands shaking to see my work. Thousands of words getting punched onto the screen while time floated. A new exhilaration pumped through my veins. That's why I write, desperate to reach that high again.