Why does my boyfriend go into flight mode/ think of worst case scenarios when there's an argument & how to help me work on that?

We had our first argument and he's the type if things get serious he goes flight mode so he thinks of worst case scenario/ end it , deal with the emotions and then he's fine. He goes fight and then flight/go cold and move on type of situation
Why does my boyfriend go into flight mode/ think of worst case scenarios when there's an argument & how to help me work on that?
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