Why are certain women not flirted with?

Men look at me but never try to approach me, flirt with me and always stand far away from me and act reserved but interact way more comfortable and even pushy with other women. I don’t expect every man in the world to be attracted to me but I also don’t think I’m super unattractive so what could be the other reasons for men to act so distant and cautious around me?

Why are certain women not flirted with?
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