Suicide is murder. It's unfair for family members, doctors, and all those involved. If the person has the ability to communicate then they are not sick enough/in enough pain to deserve euthanasia/physician-assisted death.
Though isn't holding a person's life against their will also "unfair" to the individual? A person can still talk and be in excruciating pain. Try it yourself if you want: kick yourself in the balls and talk. You can still talk, right? People suffer worse pain than that and can still freely talk. A person held alive against their will merely puts the family in jeopardy with monetary costs and "wastes time" with doctors.
Yes, depressant suicides can often be considered a greedy thing to do, but in most cases that is "easier to treat". Medication and therapy can get you through depression. It won't likely get you through Stage IV Cancer.
Still, killing people without consent is not something that should happen in any humane society.
I'd rather my family member was alive but high on morphene than dead.
I think in most cases they would ask for their family's consent, or keep it secret toward their grave to ease the pain of family members (so the doctor to lie about him / her "dying in their sleep"). Though your final statement raises an interesting question: would you rather have a person have a similar condition to morm's aunt be kept alive, possibly against her own will, than doing her and even her family a favor by *erhem* assisted suicide? I know that if one of my family members was suffering and they wanted to go through an assisted suicide (and obviously excluding depression here), I would support their cause.
On that note, I think you do bring up a good point, and I just thought of another. First:
Strange as it sounds, the ones who should die are the ones that can't give consent, as their life quality is actually zero
and then one I thought on the fly: what about individuals that are under 18? Shouldn't they also be allowed to receive this treatment? Probably under the child and parent's consent of course.