People tend to overestimate the probability of positive events and underestimate the probability of negative events, a phenomenon known as optimism bias. For example, we may underestimate our risk of being in a car accident or getting cancer relative to other people. A number of factors can explain unrealistic optimism, including self-serving biases, perceived control, being in a good mood, etc. A possible cognitive factor that has been identified in optimism bias is the representativeness heuristic (Shepperd, Carroll, Grace & Terry, 2002).
Shepperd, J. A., Carroll, P., Grace, J., & Terry, M. (2002). Exploring the causes of comparative optimism. Psychologica Belgica, 42, 65-98.