Why does AMS happen?

All is because of low atmospheric pressure. Low atmospheric pressure leads to low oxygen tension in the air which we inhale and causes a series of changes in the body which may lead to AMS. There is leakage of fluid from the blood stream into the brain. The arteries of the lungs also constrict causing increase in pressure in them which promotes fluid leakage into the lungs. These occur when our body does not get sufficient time to adapt to the change in pressure. Some people may be unable to adapt at all because of some abnormalities in their bodies, for e.g. absence of one pulmonary artery. the relative preparation their brain and the skull etc.