Anyone who is in the public eye is going to be scrutinized constantly. A popular celebrity can have influence over the younger generation because they are seen as popular and cool. Celebrities don't ask to be a role model and it isn't that "some" people see them as role models, it is simply that they become a role model by being popular in certain groups. Think of it as the popular crowd in high school, everyone wants to be like them.
I think it depends on the celebrity. Some celebrities Use drugs and do not make good role models. Then there are some celebrities that are great role models like Magic Johnson and so on. All in all it depends on the celebrity. People look up to celebrities because most have fame and fortune and a lifestyle most of us wish we had. Some celebrities are role models because we wish we have what they have so we often copy what they do.