Look at this from a cost-benefit perspective. Wearing a seat belt lowers the potential costs of driving recklessly. If you drive recklessly and get in an accident while not wearing a seatbelt, you are likely to die. If you are wearing a seat belt, you are less likely to die. This lowers the potential cost of reckless driving. When you lower the cost of doing something, more people will do it because the ratio of benefits to costs goes up.