I'm a bit rusty on fic but are the stories you're reading really saying that when character X reads specific emotion Y in character Z's eyes that X's interpretation is necessarily correct? If so I'd agree that was unrealistic but if not and it's more that X is able to form an opinion (which may be completely misjudged) of Z's state of mind from their facial expression/body language then I disagree. X would just be doing what psychologists call mentalising, creating a mental model of other people's states of mind. This is something most people start to do around the age of four and it seems so natural that we do it all the time without thinking too hard about whether our model might be completely wrong and what exactly the evidence for it is. To use a BtVS example I recall much of fandom being highly dismissive of Spike getting his soul because they were convinced from "the way JM played the part" that he had intended to get the chip out. And yet, as you point out, although his facial expression and general demeanor indicated that Spike was angry that alone wouldn't be enough to determine what he was angry about.
As to why descriptions of the eyes are so often used as indicators of the character's state of mind I think there's a good deal of scientific evidence for eyes and eye contact being of paramount importance in human communication. One example I'm familiar with is the research showing that autistic people (who are demonstrably bad at reading faces) show extremely reduced rates of eye contact during social encounters, many find it stressful and deliberately avoid look at the eyes, instead focusing on the mouth and the lower half of the face.
no subject
I'm a bit rusty on fic but are the stories you're reading really saying that when character X reads specific emotion Y in character Z's eyes that X's interpretation is necessarily correct? If so I'd agree that was unrealistic but if not and it's more that X is able to form an opinion (which may be completely misjudged) of Z's state of mind from their facial expression/body language then I disagree. X would just be doing what psychologists call mentalising, creating a mental model of other people's states of mind. This is something most people start to do around the age of four and it seems so natural that we do it all the time without thinking too hard about whether our model might be completely wrong and what exactly the evidence for it is. To use a BtVS example I recall much of fandom being highly dismissive of Spike getting his soul because they were convinced from "the way JM played the part" that he had intended to get the chip out. And yet, as you point out, although his facial expression and general demeanor indicated that Spike was angry that alone wouldn't be enough to determine what he was angry about.
As to why descriptions of the eyes are so often used as indicators of the character's state of mind I think there's a good deal of scientific evidence for eyes and eye contact being of paramount importance in human communication. One example I'm familiar with is the research showing that autistic people (who are demonstrably bad at reading faces) show extremely reduced rates of eye contact during social encounters, many find it stressful and deliberately avoid look at the eyes, instead focusing on the mouth and the lower half of the face.