Why do people get stressed at work?

Why do some people get stressed at work while others remain calm by doing the same job? Does it have to do with the level of self-confidence or experience? Is this stress the result of fear to fail, when such people feel a great (personal/internal) responsibility for doing the task in the proper way? Don't others ("calm" people) then feel responsibility in the the same way/level as the "stressed" people? Or is it simply in the character, to worry for fulfilling any task properly, even though they might understand that there's nothing to worry about?