Above is a picture of Phil Hellmuth, a rather famous poker player who a month or so ago won his 13th World Series of Poker bracelet. (He has won more than any other person.)
Now, the reason that I write about this is that poker and writing aren't that different in the end. Both are about understanding structure and about understanding players (characters). If you're not good at reading others or if you don't understand structure, you can't be a good poker player and you can't write good scripts.
Structure is really important for you to understand. It's your blueprint that you build on. It's the most important thing when it comes to planning. It allows you to use the rational part of your brain. It's also something that can be somewhat learned.
For example, in poker structure dictates what you can do. There are strategies that need to be known, like the unexploitable way to play when you have a short stack. Understanding structure gives you ideas and there's pretty much a right and a wrong way to handle certain situations.
When it comes to structure in writing, you have to know a lot of stuff. Like how to use exposition, how to introduce new characters, where the act breaks are, how long the script can go and so on and so on. It's basic and quite simple but not necessarily that easy to master. Structure is crucial to know and there's usually a right and a wrong.
But what is more important in my opinion is one's ability to read. Your ability to read characters and players. Your ability to know where you and others are so that you can react to the situations in hand.
I think this is an ability that you don't hear about enough when it comes to writing. In poker, you hear about it all the time. The best player is the one who makes the best reads. So, why aren't the writers talking about the ability to read characters that much? It might be that the best character reader is also the best writer too.
Phil Hellmuth especially prides himself on his ability to soul read players. This allows him to live in the moment and lets him make decisions based on his instincts. He usually has a pretty good idea on when to call, when to raise, when to fold and when to shove it all-in. This talent allows him to win tournaments quite often.
This reading ability, I believe, is mostly a gift. Some people have it, most unfortunately don't. Without it, you're pretty much lost. Therefore it's more important to be a great reader, because structure is something that you can learn but reading ability is something that you can't.
When it comes to writing, if you know the characters, you can safely concentrate on your plots. After all, character is the plot. So, the better you are at reading characters, the better your storylines, the better your script and the better your ability to write eventually will be.
It's that simple and I guess that's the reason why it's so hard.