Newton's Third Law
Newton's third law states: "If two objects interact, the force F, exerted by object 1 on object 2 , is equal in magnitude to and opposite in direction to the force acting from object 2 on object 1. F_12 = F_21." This is stating that a single isolated force can not exist, there is always two forces acting against each other, keeping each other in ballance.
For example, imagine throwing your fist into a wall, the harder you hit the wall (the greater force you exert), the more the wall hurts when you hit it, because it is exerting the same force back. Go ahead, try it.