Why might my friend be really awkward whenever talking about relationships?

I'm 23 and am a lesbian. I only came out a few years ago, though since then I've dated and have really liked several girls. My close friends (who are mostly all straight) and I often talk about relationships, or who we've been seeing. They don't have any issues regarding me being gay and they've always been supportive.

Though, one friend of mine always seems to become awkward and dismissive when the topic of dating/crushes arises, especially if a small group of us are hanging out. I've been friends with this girl for 11 years and we've always been close. Yet, every time she is casually asked if she's talking to anyone (which is not often), she really closes up and always says "no, not really" or "I've just been so busy with work". She's 23 like me and (from what I know) she has never liked or crushed on anyone at all. If I mention I'm seeing someone, she doesn't seem overly excited for me and won't ask many questions - she just seems uncomfortable. It makes me feel bad for talking about it.

A year ago, we were talking about what we find attractive in people, and I asked her if she has ever felt intensely attracted to anyone before. She said no. She's never been on a date and has never had even a short relationship/fling. I think she's still a virgin. I'm not saying there's anything wrong with this - there's not. I completely respect her privacy and I do not pry into her life. I'd just like her to feel she can be open with me - I'll always be here for her. She has said she's straight, and on her Facebook profile it says 'interested in men'. If she's asexual, that's absolutely okay too.

I guess I was wondering if you guys had any ideas about why she'd be awkward about this, or if you have any suggestions about what I could do to help her. I'll always do my best to understand, despite the fact I've been extremely attracted to many people in life and might not be able to relate.