Girlfriend constantly asking about past relationships. Why?

So my girlfriend of 5 months is constantly asking about all my past relationships. I tell her I don't want to talk about them, but even so she keeps asking. She has free willingly mentioned her's without me ever asking about them and will always say, "I've shared mine, so why can't you?" Honestly not to sound rude, but I feel like it's none of her business who I dated before we met, but I have never said that to her. Don't think I should have so share things about my past if I don't want to. What do you think?
Girlfriend constantly asking about past relationships. Why?
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