The exact cause varies depending on the type of arthritis you have. Osteoarthritis, the most common form, may occur due to age related changes, along with multiple other risk factors. Other forms of arthritis such as rheumatoid arthritis have multiple genetic and environmental risk factors. Psoriatic arthritis occurs in patients who have psoriasis, a skin disease that causes red, itchy and scaly rashes. So the specific cause depends on the specific type of arthritis.