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 and inversely proportional to the square of the distance between their centers.

* Inverse Square Law: The "inversely proportional to the square of the distance" part is key. This means that as the distance between the objects doubles, the gravitational force between them decreases by a factor of four (2 squared). If the distance triples, the force decreases by a factor of nine (3 squared), and so on.

In simpler terms: The farther apart two objects are, the weaker the gravitational pull between them becomes.