The exact cause of stiffness remains unknown. In general, joint and muscle stiffness seem to increase as we age. This is true whether or not we have arthritis. There are several theories to explain stiffness with and without arthritis.
The first and simplest is that we dry out as we age. The slippery viscoelastic properties of soft tissues start to decrease with time. With fatigue and aging we start to move slower, less often, and with a smaller arc of motion.
The result is an increase in the cross links between collagen tissue. In other words, the tissues stick together more and slide less. We experience this as stiffness.
In the case of osteoarthritis (OA), actual changes occur in the cartilage inside the joint and in the soft tissues around the joint. Inflammation and swelling further reduce motion in the joint. Without a smooth, freely moving joint, the viscoelasticity starts to decrease even more. The joint feels stiff with loss of motion.
Scientists don't know why some days are better than others. Changes in the weather may be part of it. The joint has sensors that respond to changes in the barometric pressure. Older adults with OA are usually good predictors of weather changes for this reason. Nutrition and hydration may also be factors.
Research is ongoing in this area as there is great interest in preventing and reducing stiffness in the aging adult population.