With humans not being optimistic, i think that the world would be a very different place. either people would be extremely successful but feeling down, or everyone would not have confidence in themselves.

People not being optimistic, means that people do not really have a high level of confidence in whatever they do, and thus, they make more than enough preparations for it, make many backup plans, in case of a failure. often, people who go the extra mile, usually succeed. However, if everyone do not have a certain level of confidence in themselves, they may be a failure in life, as they are not sure if whatever they do make pass, so many of them will just forgo what they are doing.