Here's why:

* Friction opposes motion: Friction is a force that resists the relative motion of two surfaces in contact.

* Direction of opposition: To oppose motion, the frictional force must act in the opposite direction of the intended motion.

* Parallel to the surfaces: Since the motion happens along the surfaces, the frictional force must act parallel to them to directly oppose that motion.

Example:

Imagine pushing a heavy box across a rough floor. The force you apply to the box is parallel to the floor. The friction force acts in the opposite direction, also parallel to the floor, making it harder for you to move the box.