Osteoarthritis occurs when the cartilage that cushions the ends of bones in your joints deteriorates over time. The smooth surface of the cartilage becomes rough, causing irritation. Eventually, if the cartilage wears down completely, you may be left with bone rubbing on bone — causing the ends of your bones to become damaged and your joints to become painful.
It isn't clear what causes osteoarthritis in most cases. Researchers suspect that it's a combination of factors, including being overweight, the aging process, joint injury or stress, heredity, and muscle weakness. Reference: Mayo