Why is she so secretive?

Ok so she sort of lies at times about guys in the past. I don't know if she is just embarrassed or is hiding something? And she also said she has trouble being open with someone. So I really don't get it. We are in college and have been dating for about 8 months. And I also notice that on facebook and her phone she deletes messages quite often. But its not always just guys. But I have noticed when I saw her phone one time that the conversation has pieces missing? I'm really confused?
Why is she so secretive?
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