We often look to other for social cues as to how to behave in a given situation. This is why, when children fall down and injure themselves (slightly), they often turn to their caretakers for cues. If the caretaker smiles and laughs it off, the child will too. If the caretaker screams in fear, well, you can guess what will happen.

Of course, it also works for adults, as this video demonstrates.