If the force of friction is not in opposite direction we can't walk, write etc.

Friction:
Friction is a force that opposes the motion of an object when it comes into contact with another object or surface. It acts in a direction opposite to the direction of motion or attempted motion of the object.

Explanation:
Friction is caused by the interaction between the surfaces of two objects in contact. When an object is in motion, the molecules on the surface of the object come into contact with the molecules on the surface of another object. These molecules exert attractive forces on each other, which resist the relative motion between the surfaces.

Opposite Direction:
The force of friction always acts in a direction opposite to the direction of motion or attempted motion of the object. This means that if an object is moving to the right, the force of friction will act to the left, opposing the motion. Similarly, if an object is moving upwards, the force of friction will act downwards.

This can be understood by considering a simple example. Suppose you push a book on a table with a force to the right. As you apply the force, the book starts moving to the right. However, at the same time, the force of friction acts on the book in the opposite direction, to the left. This force of friction opposes the motion and eventually brings the book to a stop. If there was no force of friction, the book would continue moving with constant velocity.

Therefore, the force of friction always acts opposite to the direction of motion of an object. It is important to note that the magnitude of the force of friction depends on the nature of the surfaces in contact, the normal force pressing the surfaces together, and the roughness of the surfaces.