If the aortic valve is narrowed, the left ventricle has to work harder to pump a sufficient amount of blood into the aorta and onward to the rest of your body. In response, the left ventricle may thicken and enlarge. At first these adaptations help the left ventricle pump blood with more force. But eventually these changes weaken the left ventricle — and your heart overall.
I'm going to look today for the technical description of what happens. It has something to do with blood supply to the heart which becomes disrupted or irregular as the heart enlarges in an attempt to compensate for the narrowed valve. This results in ischemia of the heart...which is defined as a localized anemia of living tissue. In other words, parts of your heart begin to suffer from lack of blood supply...therefore, lack of oxygen, as I understand it.
So this is why you don't want to wait to long to have valve surgery if you need it. This sort of damage is NOT reversible, according to what I've read.