Aircraft cabins are typically at an equivalent altitude of 6-10,000 ft, meaning there are fewer molecules of oxygen in any given volume.

All tissues and organs in your body needs oxygen, and the amount of oxygen that is delivered by the oxygen carrying bloods cells (red blood cells), is determined by the number of oxygen molecules in the air you breath, the atmospheric pressure the molecules are under and the concentration of the red blood cells in your blood. 

The human body is very good at adapting to some degree of altitude, but in various disease states, the ability to deliver adequate oxygen to the body at aircraft cabin altitude is diminished. This can result in significant discomfort, and physiological strain that can cause medical instability and even a threat to life.