I'm wondering if he might feel that he'll get in less trouble for saying nothing, even if he ends up having to apologize for it, than if he were to tell you something you didn't like to hear. He might, for example, think he's a lousy conversationalist and not want you to find that out till you've had more opportunity to discover his other fine qualities. He might be wishing he could talk to someone about something that he's pretty sure you wouldn't be interested in. He might might even expect you to think badly of him for being interested in it. (Quantum physics?! What kind of geek would want to talk to his gf about quantum physics?!) Or else: if he wanted to be left alone for a while to read the paper, watch the game or think about quantum physics, would he expect to be able tell you that without, say, hurting you?