Every person makes decisions based on their own experience, value system, goals and morals.
In medical decision making, a patient draws on these and also, hopefully, an analysis of the burdens and benefits of the anticipated test or treatment. The objective facts should be provided by their doctor. In a perfect world, families would have thoughtful and deep discussions about what they would do in the event of handicap, disability, choices about technology. Of course, we as health care professionals don't either, but we have pictures of the worst case and best case scenarios in our minds and families generally don't have that. Please do not think that I am advocating the opposite.