We produce less collagen; and the substance that helps our skin snap back into place (elastin and hyaluranon), becomes less springy. Environmental factors - things like sun exposure, smoking or repetitive facial movements (e.g frowning) begin to leave their marks on us. We also lose fat deposits from areas of the face such as the cheekbones which eventually causes general sagging. Muscle tone weakens as we age and post menopausal women also lose bone mass from the face.
All of these factors cause the skin to look like a balloon that has lost air. Replacing lost volume is the best way to restore the natural youthfulness of the face in a process called the "liquid facelift" or "volumising".