Why can't he look me in the eye?

There is a guy that flirts with me and another girl. They joke around and goof off whereas he likes to tease me. PLEASE give me some insight. Why is it that he is all chummy with her and can look at her and talk with her but he can't look at me. I've tried to get his attention to look at me. He'll do it for 2 seconds then look away. I'll even tease saying WHY can't you. He just gets silent and CAN'T. CAN ONLY look at me from the side when I sit down next to him. Yet with her, he has no problem. I even joked that I must have to lower my shirt to get him to look. He smiled and nodded jokingly (still looking from the corner of his eye) I started to do so (don't worry, we're both very much so adults) and he looked (from the corner of his eye) but the second that it showed too much, he QUICKLY turned his head and looked away. I know it was probably stupid to do and we were both drinking but I was SO frustrated. To flat out call him out on it and he STILL couldn't. Got embarrassed and shy! WHY?!?!?!?!? If I mean nothing to him, and she means more, wouldn't it be easy to look at me?