Hmmm...does sound wierd to me. I'll be curious as to others' responses. I would suggest you should ask him the very same questions you have asked here. It might be, that he's trying to empathize with you, so wants to appear happy if you are saying happy things and appear sad if you are sad, so you know he is sympathizing with you. Probably would be good feedback for him that he's not seeming natural but more posed and affected, like his mimicking.