What causes extreme emotional over-reactions?

This sounds like it could be a minor version of manic depressive, called cyclothymic disorder! Or she just be immature and extremely emotionally reactive. Depends on if these reactions are cyclic in nature, meaning all good then all bad. The extreme reactions suggest some sort of imbalance chemically speaking. Rational thought precludes these sort of reactions---however, she also could have severe impulsivity issues. ADHD, on the extreme end of the spectrum. Basically she may have certain brain structures that are underdeveloped. Extreme, impulsive reactions that unaffected individuals can keep quiet, she shouts out. It's an inability to regulate behavior, and it's often frustrating to individuals experiencing it. How aware is she of this behavior? If she seems to regret it, it's likely something akin to ADHD. If she seems oblivious, I'd say cyclothymia. Check into both?