Here's why:

* Motion is relative: There's no absolute frame of reference in the universe. What we perceive as motion depends on the observer's position and movement.

* Example: Imagine you're sitting on a train.

* To you, the people walking in the aisle are moving.

* But to someone standing on the ground, both you and the people walking in the aisle are moving at the speed of the train.

Therefore, motion is always defined in relation to a chosen frame of reference.