Americans are justifiably leery of political promises, whether from candidates or presidents. They have seen too many made and too few kept. The cynicism that promises generate increases the distrust of politics and politicians.
Optimism is an attitude. Overconfidence is an error in calculating statistical probabilities. As you think about the value of being optimistic, I encourage you to make a distinction between optimism and overconfidence.
Psychologists who've studied our decision-making processes have observed cognitive biases that tend to get us in trouble. These biases don't make you a bad person, they just make you human.