Why do some people struggle with opening up, like they don't open up easily at all?

she never opened up to me and I addressed it, so she said "its somethign i struggle with, but im aware of it and im working on it. you're one of the v few people i feel comfortable opening up to, so please dont be mad at me"

Why do some people struggle with opening up, like they don't open up easily at all?
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