Most of us view the world as more benign than it really is, our own attributes as more favorable than they truly are, and the goals we adopt as more achievable than they are likely to be. We also tend to exaggerate our ability to forecast the future, which fosters overconfidence. In terms of its consequences for decisions, the optimistic bias may well be the most significant cognitive bias. Because optimistic bias is both a blessing and a risk, you should be both happy and wary if you are temperamentally optimistic.

Part 1. - Bias, Blindness and How We Truly Think (2011)