Guy's Behavior

Why is this girl so quiet around me?

She is not the most outgoing girl in the world, but she will start conversations with other people (even if its just a little chit chat) she will joke and laugh with them but not with me. We have classes together and sometimes we will get together after class with other classmates and hang out and I try to approach her to talk but most of the time she doesn't seem as comfortable as she does with other people. Other times she will be a little more relaxed and even remember things I talked to her about weeks ago, like if I tell how how I dislike a certain TV show or whatever, she will bring it up later or things like that and kinda tease me playfully and argue a little with me. I'm an easy going guy, I get along with almost everybody but I don't know why she gets like that, if she was like that with others I wouldn't really think much of it but she is not and I really like her, she is smart and can be really funny and sarcastic. Sometimes I feel like a jackass trying to talk to her, I've asked her if she dislikes me or if I'm annoying her and she says no, but her actions make me think differently. Should I keep trying or what?
Why is this girl so quiet around me?
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