Pain is a signal that the body has been damaged or something is wrong.
Pain occurs when nerve fibers (pain fibers) get stimulated; the spinal cord is the main route for all pain massages to the brain, where pain is then registered.
The whole concept of pain in any joint is associated with irritation/stimulation of these pain fibers.
Joint cartilage do not have this pain fibres, so we do not have pain during day to day activities. when the cartilage gets damaged due to any hip disease , the underlying bone gets exposed and starts rubbing with each other ie. acetabulam& head of femur rub with each other so, pain fibers in this bone gets activated & patient brain gets the pain signals, & suffers from hip pain.