It is more subject to outside influences which our rational mind would immediately discount. We begin to base our confidence on the opinions of others and, more deceptively, on our opinion of the abilities of others. We may be deceived by one whose gift, like the cat which arches its back, is to seem impressive through guile. Or we may lose caution when competing against one who is easily underestimated. Thus, confidence must be tempered by rationality, the very faults of which it attempts to correct.
Confidence does have one benefit, however, which rationality can never hope to equal.
Confidence affects other people on an emotional level. Those who talk to him find that they walk away believing whatever vision he has spun. This ability to spread a net of confidence is one of the keys to great leadership. It is often the case that someone trying to fulfill a team project, a producer perhaps, will make grandiose assertions about what will happen with the project. Now, he knows that most of those assertions will not come true with the present status of the project.