According to the BBC, the major argument against human euthanasia is that it weakens a society's respect for the sanctity of life. Other concerns include setting a dangerous precedent and valuing the lives of the sick and dying less than the lives of the healthy.
The BBC explains that a common fear of anti-euthanasia advocates is that allowing voluntary human euthanasia for the terminally ill would create a slippery slope that leads to involuntary euthanasia. The slippery slope argument is based on the idea that setting a judicial precedent on one important matter opens the door for an even more extreme decision to be made down the road.
The Euthanasia.com website explains that many are worried that allowing doctors to perform euthanasia would give them a professional and financial incentive to encourage patients who are already in a vulnerable state of mind to commit suicide. The matter of consent is also a significant problem for anti-euthanasia groups as they argue that it would not be a black-and-white matter in every case. For example, the website explains that an elderly woman with dementia who is unaware of her surroundings would never be able to consent to her own euthanasia with a sound mind.