Shes just serious period. I'm a Samburu and this look is all too familiar in our women. Theres nothing sad about it. Shes probably wondering why/what the photographer is doing?
I find this one very powerful. The expression on the girl is so intense and the way she is looking at the camera almost made me feel like I could see deep into her soul...
She looks so serious... sad almost... and you've captured all this wonderfully.