Whys she such a bi***?

why is my best mate so nasty towards people, she dont like me talking to anyone else and if i do she says im better mates with them and to forget about her, she really does my head in sometimes and weve had many fall outs but we always end up mates again, i dont know what to do sometimes because she can be so horrible sometimes, if anyone writes on my wall on facebook saying something like ' thanks for a good night ' and if she werent out she'll like it to be sarcastic and as if to say thanks leave me again... but the problem is this has not just happend once, she does it all the time , if you were to look on my facebook page you could see that she'd liked everything, i feel as though im in a relationship with her sometimes about how obsessive she is over me ... i really want to know what she's thinking sometimes and whats going through her head, somebody help me?