Here's why:
* Gravity pulls everything towards the center of the Earth. Since the Earth is a sphere, this pull is always directed downwards, which is the vertical axis.
* No matter where you are on Earth, gravity will always be pulling you downwards.
* This force is responsible for keeping us grounded and for the weight we experience.
While other forces can act on the vertical axis (like the upward force of a chair supporting your weight), gravity is the only one that's *always* present and acting in that direction.
