Girls, if you are friendly and chatty with everybody, except ONE guy, what's usually your reason?

This girl I like is very friendly and chatty, smiles and giggles a lot around people. When we talk to each other I guess she is friendly because we will talk a lot, tease each other and throw heavy sarcasm at each other, but she is not AS friendly and relaxed as she is with others. I mean, even strangers will get a big smile and friendly conversation but when we talk, she never smiles (at least not as obvious) and when I try to make her laugh, she just gives me this look like saying "are you serious?", even though she will laugh at the lamest stuff sometimes. Sometimes I just feel like she doesn't like me at all but, I don't know, when we talk I can feel this strong chemistry between us, we have playful arguments that last forever and our friends seem to notice this and make fun of us. In short, with others she will be fun and relaxed but with me is is a bit serious and "mature", input will be appreciated.
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Haha I see. Well, I know she doesn't "dislike" me, because we have fun talking. She is just a bit short and serious with me. I ask her why she less "friendly" and she tells me "you'll get over it":/ lol. I try to make her laugh or smile and is next to impossible and if I'm able to do it, she'll look away or tell me I'm lame or something. Like I said, she is like that only with me. She can be a bit sarcastic to others, but she'll always smile at them to let them know she is kidding.
Girls, if you are friendly and chatty with everybody, except ONE guy, what's usually your reason?
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