Because their brains process the world around them in a different way, introverted or shy people respond differently to physical and emotional stimuli than other people, according to a new study.
About 20 percent of people are "highly sensitive," an inborn trait that can be seen in children who are reserved, need little disciplining, cry easily, ask unusual questions or have especially deep thoughts, explained Elaine and Arthur Aron and colleagues from Stony Brook University in New York and in China.
Link. This looks as if it could be interesting research. It would also explain why just telling some people to "get tough" or "ignore it" never seems to work.