Your hip is a weight-bearing joint, and all of your weight above the waistline has to be supported by your hips. Whenever you stand, walk, or run, the weight-bearing joints in your body are subjected to external forces (gravity, heel-strike shock, etc.) and the strain of repetitive motion. A runner, for example, takes an estimated 1500 strides per mile, and a force of 2-3 times the body's weight must be effectively absorbed through the lower extremities with each step. Also, the weight of the body is supported on just one limb through three-quarters of the gait (walking) cycle; during that time, the muscles in your hip must contract to keep your torso from falling to the opposite side.
Because it is a masterpiece of structural design, the human body can usually deal effectively with gravity and heel-strike shock if it is in good health and posturally stable. But if you have slight postural imbalances and/or misalignments, those forces can gradually cause a variety of painful conditions which may affect not only your hip, but anywhere from your knees to low back. Your healthcare professional can determine the cause of your discomfort and recommend treatment.