Why do people who have arthritis have stiff joints?

People with Arthritis have stiff joints because their joints are diseased and not functioning properly. Arthritis is inflammation of the joints (cause not specified). The inflammation causes the surface of the joints to become rough and for bone spurs to develop on the sides of the joint capsule. Over time, these rough patches and bone spurs start to "lock" the joint in place, reducing the range of motion of that joint.