Here's why:

* Gravity is a fundamental force of nature: It's one of the four known forces (along with electromagnetism, the strong nuclear force, and the weak nuclear force).

* Mass is the source of gravity: The more massive an object, the stronger its gravitational pull.

* Gravity acts on all objects with mass: Even tiny objects exert a gravitational force, but it's so weak that we don't notice it.

* Gravity is a long-range force: It can act over vast distances, like the force of the Sun's gravity holding Earth in orbit.

So, the more massive an object is, the stronger its gravitational pull, and the more it affects other objects around it.