You stated that Dr. Seligman "measured their success in life" - I find that assertion to be rather suspect. How is it possible to measure success objectively?

You stated that Dr. Seligman "measured their success in life" - I find that assertion to be rather suspect. How is it possible to measure success objectively? It also sounds to me like a pessimistic attitude can be very useful, especially if it guards a person against taking unnecessary risks and trying to overreach beyond his or her capabilities. Isn't it desirable to have a healthy mixture of both attitudes rather than just one?