Why is hypoglycemia dangerous?

Yes, but only. If you have diabetes and are on an incorrect dose of medication. Otherwise, there is no such thing as hypoglycemia in the absence of end-stage cancer, kidney or liver disease, or prior stomach or GI surgery. The diagnosis of hypoglycemia is made frequently and incorrectly in many people. It is not really hypoglycemia, and not dangerous but a normal body reaction if you're obese.
it can be lethal. Hypoglycemia carries risks related to worsenign glycemic control, seizures, cognitive decline, stroke permanent brain damage. I t also can result in unacceptapbe risk to those in your midst who may be mortally injured if one loses control, for example, while driving.
Brain starvation. The brain can only use glucose (sugar) for energy. Unlike most other organs in the body, it cannot use fat or protein. Glucose is only available in the blood from a recent meal, as glycogen in the liver, & as a result of breakdown of fats & proteins which takes time. Thus, hypoglycemia (low sugar) results in the brain starving. This can lead to seizures & possibly death.