What does it mean when a girl is "comfortable" with a guy?

Anonymous
So this girl seems way too comfortable calling me name and things like that (playfully) and there just a special "spark" in the air when we are together (a lot of our friends mention it). The only thing is that sometimes she seems nervous around me and quiet (especilly when we are alone). I don't know what to make of it, she just seems so at ease throwing sarcasm and teasing me (we do it mutually) and we don't have this dynamic with anybody else and its great fun but I wonder why she gets quiet and acts quiet randomly towards me and sometimes she calls me annoying but if I get quiet, she'll eventually start a conversation, any idea what's on her head?
What does it mean when a girl is "comfortable" with a guy?
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