Question

what are the causes of friction

Answer 1

there are mainly two kinda theories about the origin of friction 
1)- classical theory
    it is more of an old view. For our eyes, the surface is smooth. But when the same surface is viewed under a microscopic then we can see that the surface contains many irregularities or projections .According to the classical theory the friction arises due to the interlocking of the irregular projections on the two surfaces. But this theory was abandoned because a surface can be made smoother by polishing. If the surfaces are made extremely smooth then the friction increases enormously 

2)- modern theory
   here, the friction is due to the force of attraction between the molecules on the two surfaces at the point of contact. If the surfaces are of the same material then the force is called cohesive force and if the surfaces are of different materials the force of attraction is called adhesive force 

Answer 2

Causes of friction :-

♥ Adhesion is a molecular force that arises when two materials are brought close to each other .

♥ Surface roughness is a factor in friction when the materials are rough enough to cause serious abrasion .

♥ When one or more of the materials are relatively soft , much of the resistance to movement is caused by deformations or a plowing effect .