The simplest kind of story is where the goal of the main character is clear and all-encompassing. He has nothing else to distract him, at least not for very long. It’s all about the thing.
This kind of story is usually a genre piece, a crime, a romance, a mystery, something like that is driving the MC, and their emotional state is pretty easy to work out.
However, not all stories are that single-minded. Often a character will switch moods, or have more than one thing to deal with. And when they switch, whether because of time passing, or having to deal with different people, the writer has to transition the reader from one mind-set to another.
This is a good thing, even the most engaging stories can become monotonous if there’s no variation in tone. But if you just go from one emotional state to another without due care, it can be very jarring for the reader.