If a girl acts sarcastic and "mean" towards me all the time?

but when she does it she has this big smile on her face or pretending to be mad at me(but she isn't because I can clearly see she is holding back a smile) and saying things like "im never going to be nice to you" etc, what can I assume? She is nice to everybody else and we get along, except for all the sarcasm we throw at each other all the time.
If a girl acts sarcastic and "mean" towards me all the time?
Post Opinion