Among the people you know, who would you say shows all his emotions on his face? You know exactly what he's thinking, how he feels about what you just suggested and whether he agrees with you. He smiles, laughs, frowns, looks skeptical, roll his eyes and more. He shares his thoughts too. He tells you when he disagrees with you, when you hurt his feelings, and when he thinks you've being ridiculous. His facial expressions and body language match what he feels and thinks.
Now who among the people you know is the opposite? Who covers up her feelings, hides her reactions, and doesn't show her emotions? She may give her opinion freely (or always nod and seem to agree) but you really can't get a sense of how she feels.