為什麼會有pressure呢?

I presume that you are refering to gas pressure (or atmospheric pressure).
Gas pressure is due to bombardment of gas moelcules because of their radom motion. During each impact, there is a change of momentum of the molecule and thus a force is exerted. The total impact from the molecules constitute a pressure.
From the kinetic theory of gases, the pressure is expressed by
p = (1/3).d.c^2
where p is the gas pressure
d is the gas density
c is the root-mean-square speed of the gas molecules