He is observing her face for social ques to determine how he should act and respond and how he should look with certain emotions.
It makes sense if he personally struggles with feelings and emotions, and since she is very much like an open book emotionally, he can "read" or understand her better than most people. Plus she is easy to please. And I think some part of him wants to be normal and accepted. I feel like its not so much about him liking her, but him trying to keep her around by keeping her happy so that she doesn't leave.