You see, I am a fan of it, in theory, but I fully understand why, in the UK at least, it isn't legal.
Reason 1: There is a danger that family members who wish for inheritance or simply find the ailing person too troubling will pressure the ill person to suicide.
Reason 2: If the person is unable to talk, it means that the power of life and death may be left with someone who has opposing views on such matters than the person. Death isn't reversable (until I discover my Beyond Two Souls machine)
Reason 3: It may ask medical personal to act against personal beliefs, and there are interpretation of "do no harm" which include for some doctors, suicide.
Reason 4: Once we start agreeing life can be cut short actively, our publicly funded NHS may decide that "Oh, your life is too expensive to keep, so we're going to cut it short before it gets REALLY expensive!" It's already capped at around £30,000 a year.
Reason 5: There is a moral assumption that the lives of the sick or ill are worth less than others if we limit it to only those, or we are going to open it up to everyone?
Reason 6: What is mental competency? If we simply euthanize everyone who said they wanted to die, Tumblr would be empty in days.
Reason 7: If people become ill, legalising it will let people start treating those who are ill worse, as there is no longer the need to try and treat them properly, as well as reduce the need to comfort the dying, who are simply "not doing it efficiently"
Reason 8: People who are estranged from family may feel it as their only option.
So for the above reason, I feel it would be far too difficult and messy to regulate properly, and that it would open up a whole mess of legal, moral and philosophical problems to be ever usable in the grey shaded human society.