Why do I feel like I've disappointed her? I'm sorry Rei for whatever I've done.

I thought this was her happy face.

she looks very happy... and cute. I love how the slightest changes can show Rei's expression.

Well, if she's disappointed or bored or disinterested, she'll look sideways or avoid eye contact.

Her paying attention to you means you are on the plate.

if her "smile" was drawn a couple of millimeters narrower, that would be her disappointed face.