Why are girls nice to me when we're alone but mean when we are in a group?

This has happened to me in middleschool, highschool and now it's happening in college: whenever i'm alone with a girl they are friendly and polite but when other people are around they start making fun of me, making mean remarks on everything i say, etc. And i've seen them making fun of other guys but only behind their backs, it's like i'm the only one they insult straight to face.
Why are girls nice to me when we're alone but mean when we are in a group?
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