I can't speak for the other person, but the eyes are one of the best indicators to understand how an individual is feeling, because it's nearly impossible to control those reflexes.

They widen in surprise, people look in a certain direction when they're recalling a memory and usually in the opposite direction when fabricating a lie, pupils can dilate due to fear or arousal...

[ She lists various examples linking optical reactions to psychological stimuli in a clinical "do-I-really-need-to-explain-this-to-you" sort of tone. ]