It's probably because she hasn't thought about her beliefs. She just knows she has faith. She doesn't know why and can't explain it to you, which is probably why she is uncomfortable talking about her beliefs. I suggest talking to her about faith. I do not suggest trying to convince her she doesn't have faith. She must ultimately come to that false conclusion (in my opinion) on her own. Try talking logically about your own beliefs, that might be a good starting point.
It may also be because she doesn't want to sound preachy. Discussions about beliefs have a way of spiraling into big fights. People tend to believe that they are right, talking about belief with someone who disagrees with your belief usually becomes a battle of wills.
It may also be because she doesn't want to sound preachy. Discussions about beliefs have a way of spiraling into big fights. People tend to believe that they are right, talking about belief with someone who disagrees with your belief usually becomes a battle of wills.