Here's why:

* Newton's Law of Universal Gravitation: This law states that the force of gravity between two objects is directly proportional to the product of their masses. This means that if you increase the mass of either object, the gravitational force between them will increase proportionally.

* Inverse Square Law: The force of gravity also depends on the distance between the objects. The force decreases rapidly as the distance between the objects increases (following an inverse square law). However, the effect of mass is still more significant in determining the strength of the gravitational force.

In short, the more massive the objects, the stronger the gravitational pull between them.