The higher up you go, the thinner the atmosphere gets. That means breathing in the same amount of air gets you less oxygen than at a lower altitude. Altitude sickness happens when your body has trouble adjusting to the difference in how much oxygen you're getting with each breath.

Altitude sensitivity, or Acute Mountain Sickness (AMS), occurs because the atmospheric pressure drops as you go higher, meaning there are fewer oxygen molecules in each breath. Your body’s ability to adapt depends on several factors, including genetics, your baseline fitness, and how quickly you ascend. Some people have a lower "ventilatory response," meaning their lungs don't automatically speed up enough to compensate for the thin air. Dehydration also plays a massive role, as high-altitude air is very dry, and your body loses water through rapid breathing. Additionally, if you have underlying conditions like low iron (anemia) or certain heart/lung issues, your blood has a harder time transporting the limited oxygen available. Age and physical fitness are surprisingly poor predictors; even Olympic athletes can suffer from altitude sickness if they don't allow 24–48 hours for their bodies to produce more red blood cells through acclimatization.