What does it mean when my girlfriend says that she won't marry me?

We are really young but when i joke about it or send her marriage tiktoks, she always says no. I do 100% know that she loves me deep in her heart. Why would she say that? Can you help me understand why does she does it? I tried to talk with her and she said that she does not plan to marry her first serious partner (serious relationship partner), it really hurts me deep :(

What does it mean when my girlfriend says that she won't marry me?
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