Surely we know enough about humans to remember that decision-making is an incredibly complex affair, especially in regard to future health care. People are ambivalent about everything almost all of the time, and especially about matters of life and death. People are susceptible to pressures in and around them, including subtle suggestions and innuendos that they not not even acknowledge to themselves, let alone voice to anybody else. Dying is one of life's most important occasions.
Time is necessary to give blessings, make reconciliation and say good-byes.
Most people have second thoughts about any decision they have made in the past, especially when confronted with the reality of what is just about to happen to them. Assessing mental capability is extremely difficult. Few people can make a completely rational decision. Those who have been abused expect to die young and survivors often feel they deserve to die. Because of these factors, ancient medicine included oaths that provided immutable guidelines, so that they were not susceptible to family squabbles, politically correct morality, economic pressures or even the whims of patient choice.