Here's why:

* Friction arises from the interaction between the surfaces at a microscopic level. The surfaces are never perfectly smooth, even if they appear so to the naked eye. There are microscopic bumps and irregularities that interlock when the surfaces come into contact.

* Relative motion refers to the difference in velocities between the two surfaces. When one surface tries to move relative to the other, these microscopic interlockings resist the change in their relative positions, creating a force that opposes the motion.

Example:

Imagine pushing a heavy box across a rough floor. The box's surface and the floor's surface are in contact. When you push the box, you're trying to create relative motion between the two surfaces. The friction between the box and the floor will oppose this motion, making it harder to push the box.

In simpler terms:

Friction doesn't care about the actual motion of each surface, it only cares about the difference in their motion (the relative motion). If both surfaces are moving together at the same speed, there's no relative motion, and therefore no friction.