Quick context: About a year and a half ago, my girlfriend cheated on me with a coworker and over time after we broke up we became close friends again. I asked some probing questions about what exactly it was about him that she found so attractive despite dating me, and although she had a few different vague answers, one specific thing she kept saying that sticks out to me is this: The FIRST thing that caused her to start finding him hot (and not just another kinda cute guy she knew) was the fact that he often refused to answer her questions. She's someone who likes to ask people a lot of very personal, deep questions about themselves when getting to know someone, and he would sometimes respond by refusing to answer the question, saying something like "wow. I'm not answering that" or whatever. I asked her if she knew why she found that so attractive, and she couldn't really give me an answer.

Anyways, this is very strange and counter-intuitive to me. I'm not sure why it would be a good thing to be such a "closed book", especially considering just how attractive she found it. I can guess at a few possible explanations (shows firm boundaries, makes him more mysterious, etc), but I feel like there's something here I'm missing.