Hi
It was observed that every object applies force on every other object. Then it was made a law, a gravitational law . Law is somehing which is always valid , for eg, newtons law are mere observations and u cannot prove them or contradict them.
Likewise, Gravitation is a universal phenomenon, that exist and is in the nature of each object that posseses mass. Cause of this force is the mass of the object , and no reason for why there is a force, its a universal fact.