When she is being negative by being bitchy or dramatic, or perhaps argumentative. It is typically the guy's fault for shaping her behavior that way. All people have negative sides to their personalities, but you are bringing out her worst side.
You react to the undesirable behaviors and criticize or confront her on them. This is counter productive and causes her to do them more.
Humans are stimulated and rewarded when people give them attention. It doesn't matter whether it is negative or positive attention. So if you give your attention to a girl who is not acting in a way you like, she will just do it more.
Try ignoring and just taking your attention away from a girl when she is doing something you think is undesirable. This will motivate her to stop it and bring out her better sides.