Sometimes I think it's a shame that I'm not a professor of writing, because I have so many lessons to impart that were never taught to me. The biggest amongst these is that a work of fiction (particularly a novel) cannot stand unless you've discovered the heart of longing.
In most cases, this longing will be the character's longing: the desire that animates their action in the novel. But not always. Sometimes the longing suffuses the novel's narration. In any case, I think each writer has to find their own path to the heart of longing. Moreover, it's not something you capture once and then possess for all eternity; it's something you need to find again and again.
The way I'm writing this advice, it probably sounds obvious to you, but I would say that most unpublished and apprentice works tend to lack the heart of longing. That's because it's very easy to write something that resembles a novel--something with a lot of verve and action--which is not propelled forward by the character's own longing. For instance, if you write a book about someone being chased by a wolf, it's very easy to write a book in which the character runs and fights and runs again, but never really exhibits that desperate desire to survive that would, in this context, constitute the heart of longing.
The heart of longing isn't necessarily something you explain, though. It's something that suffuses the work. I'm thinking of the movie Dunkirk, for instance, where none of the characters have any backstory, but they all exhibit such an immense desire to live, and this desire pushes them to do and attempt things that are unexpected and unusual.
Another example is the movie Gravity, which could very easily have been a "man is chased by wolf" story, except that Sandra Bullock's performance was so magnificent: at each stage she dramatized the decision to fight onwards vs. give up.
Which is to say that it's extremely difficult to point to any part of a book or film or story and say, "Here is where you need to add the heart of longing." Instead it's something that needs to suffuse the entire work. In fact, you probably shouldn't even begin to write unless you've got the heart of longing. (Conversely, if you do have the heart of longing, then you should keep revising, even if the novel seems otherwise hopeless, because having the heart of longing is fifty percent of the battle).
Note that grasping the heart of longing isn't enough by itself to get your work published. You need other stuff too. In fact, if all you have is longing, then people tend to be turned off. They feel like your character is too desperate. They need to not just have longing, but also to be in some way larger than life and heroic. It's a tough thing to manage...