Because of the higher elevation. As much as we go away from earth by increasing altitude, there will occur lacoccuren. due to this, we need to breathe fast to get suffient oxygen by time at top of the mountain. We breathe faster at the top of a tall mountain due to a natural reflex. As we go higher up a mountain, the density of air and oxygen decreases, i.e. the amount of oxygen in the air decreases along with other gases. Therefore, to maintain the regular supply of oxygen, the body breathes faster. If we go too high without oxygen tanks, we could fall unconscious and die within 8 minutes due to oxygen deprivation.
Hope this helps! let me know if you need anything else!!
