What does it mean when a girl keeps to herself?

Does it just mean she's quiet or shy? I was talking with a friend of mine and I'm not sure if he's right but he said that a lot of girls who are like this usually are with abusive men or maybe had an abusive childhood. I'm not sure if he's right, but he's been with a lot of girls so I guess he knows more than I do. lol What are your thoughts?
What does it mean when a girl keeps to herself?
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