I watched my grandfather succumb to the pain, cold, and discomfort of lung cancer, and I could see him suffering every day.
Still, if had he asked me to kill him, I would never have done it, no matter how much he begged, yelled, or screamed.
While I am not opposed to euthanasia in rare, unusual circumstances (like Terri Schiavo), for the most part I am against it. As much suffering as life contains, the breath of life is not something to deprive of someone simply because they no longer want to live. A terminal illness may be debilitating and painful and a living hell, but at least you are alive. Once you're dead, there is no return to this world, and I'd refuse not for my sake but for the sake of the loved one, I'd ask that they live and fight, even if there's no hope. That's what living is.