Which best explains why some people are more strongly motivated to save money than other people?

Some people have different viewpoints on what constitutes "risk" and they see the future in varying degrees of such. Some people worry that there may not be enough money for themselves or their families in the future, so they want to ameliorate this risk by saving up now, while others see less risk in not having funds in the future, so they are less likely to want to save at present.