The word optimism is from the Latin word "optimus", which means the best. This word represents the world where the positive dominates negative, but this world has advantage and disadvantage. Opposite to optimism is pessimism.
A lot of people take seminars and read books about optimism because optimism has many good things that people want to learn. Optimism helps some people to get out of depression and improve their physical health. People who are optimists do not believe in bad things. They usually have a sense of security about the future because they believe that next day their life will be happy and without problems. Optimists change bad and ugly things to good and wonderful. For example, when I came to the USA, I lived in a poor and uncomfortable apartment. I began to tell myself that my home was not bad and many other immigrants have worse apartments than I. Inside myself I understood that it was not true, but after cup of months I adapted to this thought. Now, I think that my apartment is very nice. In my opinion, if I can't change things surrounding me, I try to change my attitude toward these things.
The disadvantage of the people, who are optimists, is that they sometimes do not recognize difficult problems because optimists live in a world where, they think, life is kind and without barriers. They may not be ready to solve their problems because optimists do not expect problems. In order to get the best result, optimists need to use some pessimism because pessimists look their problems more seriously. Optimists are more vulnerable than pessimists are. So, for a successful life people need both pessimism and optimism because they can help balance each other.
However, if optimists are too confident and begin to deceive themselves then they begin to believe in their own self-deception. Usually this form of optimism has negative influence on people. If some one believes in things that he or she would not be able to reach any way, their personality will be destroyed and their lives are frustrated.