The body is designed for survival. In order to trigger muscle growth, you must engage the body's survival mechanism. When the body is asked to lift heavier and heavier loads, it perceives this as a threat to survival. The human body is genetically programmed to build muscle mass in order to handle future threats and thus guarantee survival.
The important thing here is to remember that you must force muscle to get bigger. It won't get bigger unless it is stressed, rested, stressed more, rested, stressed even more, rested, etc.