It would rarely be a truly free choice. It would be very difficult to obtain valid informed consent. It is rare to find a patient with a fixed, rational, enduring wish to die. Fear of pain, rejection, loneliness and lowered self-esteem because of dependency are all variables that can influence the patient's wishes. As these needs are met the desire for death recedes.
If a patient is depressed or in pain, informed consent is not possible and the physician may not be able to assess depression in a patient he does not know well.
The result could be that a goodly number of patients would be killed who didn't really want it. Uncontrolled depression and pain are thought to be the two most frequent reasons why patients seek euthanasia. Both can be controlled so that the reason for seeking euthanasia vanishes.