Here's why:

* Gravitational force is due to mass: Gravity is a force of attraction between any two objects with mass. The more massive an object, the stronger its gravitational pull.

* Symmetry and cancellation: At the Earth's center, you are surrounded by an equal amount of mass in all directions. The gravitational pull from each part of the Earth cancels out the pull from the opposite part. Imagine yourself at the center: you're being pulled equally in every direction, resulting in a net force of zero.

Important Note: This assumes a perfectly spherical and uniformly dense Earth. In reality, the Earth is not perfectly spherical, and its density varies. However, the concept of the gravitational field being zero at the center is still a good approximation.