I think it depends on the person. It can be just because of your personality, or maybe something has happened in the past that made you introverted.
For me, it’s the second one. I used to be a talkative child who always made friends wherever I go. But, I eventually got bullied, and that made me introverted and made me have social anxiety.
It’s better now. I still can have moments of anxiety whenever it comes to talk to people or when I am at a crowded place and I still prefer to be alone, but I’ve had some sort of therapy which made me more confident 
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