Why does one of my best girl friend's act like the biggest in the world? I can't talk to any girls!?

I've known this girl for 4 years and we are very good friends (just friends!). We talk to each other about a lot of stuff, my girl problems, her boy problems, it's what a platonic friend would talk about. The thing is that whenever I try to flirt with other girls she goes into super flirt mode and tries to ruin my game! I don't think she genuinely "likes" me, so why does she always barge in when I talk to other girls?