Why don't girls get along with girls?

People always say being a girl is easy but those people have never been girls. I do not hate girls, or at least not intentionally but sometimes I cannot help but feel that certain girls are desperate to compete. Even though, most times, there really is nothing to compete for. 

Research have shown that mothers have more problems with daughters as compared to sons. Hard to believe, isn't it? 

Even in movies you always see daughters fighting with their mothers over tiny things. More often than not, the argument will escalate to the daughter stomping off, screaming "I hate you."

So why the animosity? There is one girl in my life that I do not see much of but whenever I do, she only has mean things to say. She hides her malicious comments behind girly giggles but I am sure deep down inside she means what she says. I have done nothing bad to this girl and neither have I had an argument with her in the past.

She is beautiful, rich, and she has a good and some may say envy-worthy career and lifestyle, yet it seems to me as if she is jealous of me. I have nothing compared to her. Which leads me to the question, why be jealous of somebody beneath you and your obvious success? 

Is it a girl thing or is it insecurity? You tell me.