I am going to answer this question based on the bio social theory. The theory explains how the combination of a genetic predisposition and a specific social environment can manifest into maladaptive behavioral responses.
Research on the brain shows that some people tend to experience things more intensely, and as a result are more reactive to events than other people. This can be something that is part of the person’s genetics or the product of early life experience. Either way, the person has a higher degree of sensitivity to emotions, so that they are activated more easily, heightened in their response, and slower to come back down once activated.
It is important to note that it is not just the person reacting more intensely. That could sound like blaming! It is that their experience is more intense for all events, both pleasant and unpleasant.
Emotional sensitivity is not a problem in itself, In fact it has many benefits, such as increased intensity of love, passion, empathy and connection. However, when a person does not know how to take care of their sensitivity they may learn to dull the pain through escaping or avoiding emotions. This can lead to extreme behaviors that have their own consequences (ie cutting, substance use, eating disorders), as well the by-product of maintaining the belief that the person cannot tolerate and cope with intense emotions.