Why forces come in pair? Can a body 'X' apply gravitational force On a Body 'Y' without being itself acted by a gravitational force By 'Y'?
 A: You are essentially asking why Newton's third law is true, if I understand you correctly. That law says that if an object exerts a force on another object, that second object exerts an opposite force of equal magnitude on the first object.
We could say that there's no reason why. It's just the way nature is (which is what all of physics boils down to at some point). But we can go one step further and say that Newton's third law is a way to guarantee conservation of momentum. The more fundamental fact is that momentum is conserved. Force is the rate of change of momentum. If the momentum of one object changes, then another object's momentum has to change in the opposite way in order to conserve total momentum. So this object must experience the same force (in magnitude) as the first object, but in the opposite directions.
Newton's third law states that this second object is always the one which exerts the force on the first object, instead of some random object halfway around the world.
