"For the vast majority of people we don't have a good explanation why it occurs." Doctor Thomas P. Sculco, Surgeon-in-Chief at the Hospital for Special Surgery in New York, explains several common causes for arthritis, while noting that it's still a relatively mysterious condition.
Dr. Sculco: Arthritis just means inflammation of a joint. And that inflammation can occur because the joint has become deteriorated. Arthritic. Worn. Or it could just be inflamed. There are forms of arthritis that occur after an injury, for example. Others are secondary to a genetic pre-disposition. A mother will have it, a grandmother will have it. More common in women, when it's a genetic component. There are sports injuries in men, for example, that repetitively traumatize the hip and can cause arthritis. For the vast majority of people we don't have a good explanation why it occurs. The theory is that the joints wear out as people get older. But that's probably not a great answer because they are a great many people in their 80s and 90s that don't have hip arthritis. So it's probably multi-factorial.