Pets age. They grow weak and old just like us human beings. There is deterioration in their ability to hear and see. Mobility is hampered and skin and hair condition degrade.
Ageing causes unavoidable illnesses like arthritis, gastrointestinal upset, cancer, heart, kidney/urinary tract and liver disease, diabetes and senility along with weakness. They are often irreversible and fatal.
Many pet owners panic once their pet is in a challenging position. The expenses, time, work and stress involved in the healing or maintenance often leads many owners to shirk the responsibility and opt for euthanasia.