Girls, why would a woman feel guilty of her past?

My girlfriend is 28 years old (10 younger than me) and she has been with more than 50 sexual partners during her life (probably many more, I don't know), and at 20 years old she had already accomplished all of her fantasies and by 25 years old she had done everything related to sex.
From the other hand, I started having sex at her actual age, 28 years old, and I had no special sexual experiences.
I love her with all my heart and her past doesn't bother me (even details like, p. ex., the fact that before being with me she had sex with our coworker and she also had sex some years ago with another guy I know). On the contrary it even excites me that she is so experienced, having enjoyed so much and got so satisfied before meeting me, to the point that thinking about all that during sex, pushes me to give my best to her.
However, I still feel like she is a bit ashamed of her past. I keep telling her that it was nothing wrong with that, and I honestly believe that, because I love her no matter the number of her sexual partners, but yesterday she told me that if when she was a teen would know that in the future she'd have this relationship with me, she would have saved her virginity for me. I found that the most beautiful thing said by any person to me, but at the same time I feel really sad that she feels guilty about her experiences.
Girls, why would a woman feel guilty of her past?
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