There are a number of different reasons we have joint pain. Working a joint too hard can cause irritation in the cartilage, the  stiff but flexible connective tissue that  coats the surfaces of the bones in our joints and cushions bones against impact , as well as inflammation.   Inflammation is the body’s natural defense mechanism to any foreign substance that it considers harmful. Germs, pollen, and chemicals may trigger an inflammatory response when the body is exposed to