Here's why:

* Gravity is a universal force: Every object with mass exerts a gravitational force on every other object with mass. The more massive an object, the stronger its gravitational pull.

* The Earth's gravity is strong because it's massive: The Earth is very large and has a lot of mass, which is why its gravitational pull is strong enough to keep us on the ground.

* Other celestial bodies exert gravitational force: The Sun's gravity holds the Earth in its orbit. The Moon's gravity causes tides on Earth. Even smaller objects like a rock exert a gravitational force, just a very tiny one.

In short, gravity is a fundamental force of nature that acts between any two objects with mass.