Why won't my Girlfriend tell me what's wrong?

Sometimes I see my Girlfriend have tears in her eyes or she acts and speaks differently, whenever I ask her "What's wrong?" or "Are you ok?" She responds with "Nothings Wrong." or "I'm fine.". We have known each other since grade 3 and she can always tell me, she chooses not to.
Why won't my Girlfriend tell me what's wrong?
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