Which factor is a primary cause of the gravitational force between two objects according to Newton's Law of Gravity?

Understand that Newton's Law of Universal Gravitation describes the gravitational force between two masses. The formula is: $F=\frac{Gm1m2}{r^2}$, where $F$ is the gravitational force, $G$ is the gravitational constant, $m1$ and $m2$ are the masses of the objects, and $r$ is the distance between the centers of the two masses.