What does the girl mean by not giving her an ambiguous expression? He is not used to it.

Because he thinks you are a good person, and he may be seeing someone else. He doesn't want to get involved with you but he doesn't want to lose you as a friend. So he chose to change the topic and become a real blue face with you. Trust me, don't flatter me too much. As the saying goes, nothing in the world is a good deal. You have to make him pay attention to you, or you will all. ...