Final answer:
An animal with osteoarthritis experiences pain due to the breakdown of cartilage, bone damage from increased joint pressure, and inflammation, which can cause significant joint pain and stiffness.
Step-by-step explanation:
An animal with osteoarthritis feels pain because this condition involves the breakdown of joint cartilage and underlying bone, leading to joint pain and stiffness. Osteoarthritis is thought to be caused by mechanical stress on the joints alongside insufficient self-repair capabilities of the cartilage, often compounded by low-grade inflammation of the joints. As the cartilage deteriorates, bones may eventually rub directly against each other, causing damage to the articular surfaces and significant discomfort. Movement aggravates the pain and can lead to decreased activity and subsequent muscle loss.