Here's why:

* Newton's First Law of Motion: An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. This means that for an object to move horizontally, there needs to be a force pushing or pulling it in that direction.

* Horizontal Forces: These forces can be various:

* Pushing: Applying a direct force with your hand or a tool.

* Pulling: Using a rope or string to exert a force.

* Friction: Can act in the opposite direction of motion, slowing down the object.

* Gravity: Acts downwards, but if the object is on a slope, gravity can have a horizontal component.

* Wind: Can exert a horizontal force on objects.

Example:

* If you push a box across the floor, the force you apply is the horizontal force causing the box to move.

* If you roll a ball down a hill, gravity's force has a horizontal component that makes the ball move forward.

It's important to note that friction often opposes horizontal motion, but it's still a horizontal force.