Here's why:
* Friction opposes motion: Friction arises from the interaction between surfaces in contact. It always acts in the direction that opposes the relative motion between those surfaces.
* Examples:
* Pushing a heavy box across the floor: Friction between the box and the floor acts to slow down the box's movement.
* A car braking: Friction between the brake pads and the rotors converts kinetic energy into heat, slowing the car down.
* Air resistance: Friction between a moving object and the air molecules acts to oppose its motion.
There are other forces that can oppose motion in specific situations, but friction is the most general and common force that directly opposes motion.
