Suppose you press your physics book against a wall hard enough to keep it from moving.
Does the friction force on the book point into the wall, out of the wall, up, down, or is there no friction force?
The frictional force acts upwards, so as to oppose the downward force of gravity. The reason that pressing against the wall harder makes the book stick more is because the reaction force from the wall increases and frictional force is directly proportional to reaction force.
