'Wear and tear' is the rule for all things that are put to any kind of use. The same holds good in case of joints too. They function longer if you take good care of them, yet they do ultimately show up some or the other signs of being worn out.

Degeneration of the cartilage (the cushion) between the bones (that form a joint) is the primary cause of osteoarthritis. As the degeneration progresses, the cartilage may be completely lost leading to rubbing of the bones during the joint movement. This leads to pain and the grating sensation during any movement.

There can be numerous reasons why the cartilage wears off:

As a part of the ageing process

Joint injury, repetitive trauma

Hereditary

Muscle weakness

Being overweight (causes increased pressure on weight bearing joints)

Abnormality of the joint anatomy (e.g. being bow legged)

Presence of Gout, rheumatoid arthritis, septic arthritis or any other disease that affects bones or joints

Diabetes

Growth hormone disorders

Inappropriate footwear (causing increased and imbalanced pressure on the knees)

In cases where there is a known cause that induced the problem, the condition is called Secondary Osteoarthritis. Cases which have no known cause are termed as Primary Osteoarthritis.