Why does he belittle himself?

Why does this boy i like belittle himselves in my presence? Like if I pay him a compliment he replies by pointing out his flaw. He does it ironically, but to me it seems like a defense mechanism, I don't understand from what. He knows I'm interested and sometimes we really flirt, but I feel like he doesn't give a damn because he's scared of something. He's very wary. What do you think about it?
Why does he belittle himself?
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