When one lifts a block off of the ground and holds it in the air, the work done by one's lifting force is equal to the change in total mechanical energy of the block/earth system. Since the mechanical energy is KE+PE and since KE doesn't change during the motion, the work done by the lifting force is equal to a change in PE. This is obvious and is easy to visualize from an energy transaction perspective: the biochemical energy in the arm muscles gets converted into potential energy. However, when one lowers a block from a shelf and places it on the floor, the work done by the lowering force is equal to the change in total mechanical energy and that change is negative since the KE is constant and the PE goes down. Since the mechanical energy of the block/earth system went down and since all energy is the universe is constant, where did that energy go? If the block/earth system's PE went down, something else's energy must have gone up. Did the thermal energy in the bones joints of the person lowering the object go up? Did elastic energy in the atoms of the person's body go up due to the weight of the object? Clearly, the person's biochemical energy did not get replenished by the drop in potential energy (which would make no sense), but what form of energy did go up?