Let's say you have an object on the ground. Why do they say that friction is proportional to the normal force instead of saying it's proportional to the force of gravity? I know that they're equal in magnitude, so you get the same answer, but they're opposite in direction. The force of gravity exerted by the Earth on the object pulls the object TOWARD the surface of the ground, whereas the normal force exerted by the ground on the object pushes the object AWAY from the surface of the ground. It makes more sense to me intuitively that the force of friction between two surfaces is proportional to the force that pushes or pulls two surfaces together.