Here's why:

* Forces cause changes in motion: Forces can cause an object to start moving, stop moving, change direction, or change speed.

* Balanced forces cancel each other out: When two or more forces act on an object in opposite directions with equal strength, they are balanced. This means the forces effectively cancel each other out, resulting in no net force.

* No net force means no change in motion: Since there's no overall force acting on the object, it will remain at rest if it was already at rest, or continue moving at a constant speed in a straight line if it was already moving.

Example: Imagine a book sitting on a table. The force of gravity pulls the book down, but the table exerts an equal and opposite force upwards. These forces are balanced, so the book remains stationary.