Here are some additional points to consider when you are trying to make this kind of decision.
Several factors contribute to quality of life, all having their influence. Knowledge of these factors is important for a pet owner trying to make a decision of continuation of life versus euthanasia.
Social relationships—Dogs being highly social animals, a set of pleasant and unpleasant emotions promote and cement social relationships. Positive social affiliations and companionship elicit pleasant emotions, and separation and isolation elicit unpleasant emotions (e.g., loneliness). These emotions have strong influences--positive and negative on quality of life. Do you have to leave your pet isolated due to incontinence, aggression or other factors? Is this isolation permanent? If so, are you able to provide your pet with sufficient interaction to support enough happiness?
Health—It is the immense importance of health matters that explains why Quality of life is often mistakenly equated to health status. Medical conditions impact quality of life through the unpleasant situations associated with illness and injury. Nausea, pain, weakness, pruritus (itching), and inadequate oxygen intake are examples of these factors. Physical disabilities limit one's opportunities for experiencing the attainment of pleasure (for example, by preventing the dog from being able to play ball or hike in the mountains like he used to), thereby negatively affecting quality of life.
Food consumption—The pleasant taste of food and the unpleasant feeling of hunger both motivate consumption of nutrients to sustain life. When a pet no longer has an urge to eat, it is a pretty reliable indicator that his quality of life is greatly affected.
Stress—Stress primarily refers to specific unpleasant emotions, such as fear, anxiety, pain, loneliness, boredom, and anger and are the result of the encounter of situations perceived as being non survival.
Your happiness—Your happiness in the presence of your pet and that of your family has a great influence on when to make the decision to euthanize your dog. Has it become too stressful, too demanding, beyond your financial and time wherewithal to keep your pet alive? This is a question that only you and your family can answer. It is not fair to you, your family or your pet to keep a pet around when doing so hinders everyone's happiness including the pet's. When your happiness is affected, when it has become stressful to care for your pet, don't deceive yourself: your pet will feel your unhappiness and although he or she may not understand its origin, his or her own happiness will be affected by this change, no matter how much you try to hide it. Pets, especially if you have a deep emotional connection to them, perceive this stress and unhappiness very well.
Other pets—Sometimes, the issue arises in a multiple pet household that other animals can become “mean” to a sick or debilitated pet. It can become distressing and puzzling to a pet owner to witness the healthier pet(s) “pick” on the sick or old one. As human beings able to demonstrate compassion and help others who are weaker, it can be difficult to understand how such behavior is acceptable and even desirable in the animal world. Animals instinctively sense when another becomes weaker and they may consider the weak or old pet as a threat to the survival of the pack. They may simply ignore the weakling or go as far as growling, snapping or injuring him or her whereas before they were friends. Cats, not being so gregarious, don't seem to display this kind of behavior as much. However, it is possible to witness such behavior in cats also, particularly when a pet comes back from the veterinary clinic and has all the smells of the hospital on him or her. Other cats in the household can become aggressive to the pet who is returning from a visit at the vet's. This may be a temporary reaction. However, if the aggression of other pets continues and is due to the weakening of the pet, it may become a factor that will influence a pet owner's decision as to when “it is time”.