Question

while firing a gun man has to hold the gun tightly against his shoulder why

Answer 1

because due to recoil the gun will come back with a velocity that may break our shoulder bone

Answer 2

according to netwon's third law of motion,  "For every action, there is an equal and opposite reaction." so the bullet leaving the gun is the action and the recoil of the gun is the reaction.

A gun is tightly held on the shoulder when it is being fired because when bullet is fired the gun recoils backward and this backward push is so hard that it can displace our shoulder and can cause a sever injury to us. Placing it on the shoulder decreases the impact of recoil.