Great book on the psychology on/around project management. A few quotes and snippets below.
Humans have the gift of overcoming their own uncertainties with objective arguments and self-construed facts which are then consistently believed by them. If that faith releases a significant amount of energy, it might enable people to actually reach their goals to the extent that was achievable in the first place.
If the prognosis does not come to pass people rarely tend to doubt the facts as they have construed them. They rather seek the reason for the failure beyond the realm of their own responsibility. They find ample opportunity in the form of projecting onto other people, pointing out external conditions, or rationalizing with seemingly reasonable explanation.
This is one of the hardest lessons to learn in business. Clients typically will want everything they can get, as I would totally expect and do when I am the client. Sometimes, however, no is the correct answer.