Question

why do u need to breathe faster at the top of tall mountain

Answer 1

At the top of talk mountain the air is thin that is the oxygen content reduced therefore to meet the oxygen demand one breathes heavily.

Answer 2

Our body needs energy to function, which is released after combustion of food in the cells. For combustion, we need oxygen and as you know, the pressure drops as we go higher in altitude and gets higher as we go deeper (that's why our body will get crushed if we go deeper into the sea). So the pressure drops on a high mountain, and thus, the air gets thinner as individual air molecules occupy larger volume than molecules at normal altitude. Since the air is thinner, oxygen molecules for our  breathing are low too. This is the reason we breathe faster at the top of a hill/mountain. Our body needs more oxygen to function but the required amount doesn't reaches the lungs. So we breathe faster and faster to increase the intake of more oxygen.
Hope it helps!
HARRY POTTER FOREVER! XD