If you look at the surfaces of materials in microscopic scales, you will find that the even the smooth surfaces are not smooth. There are many ups and downs in the sufaces which cause the interlocking between surfaces when two surfaces are in contact. The amount of ups and downs in different surfaces are different from each other. In some surfaces, the locking is better and hence the friction is more and in some surfaces the locking is not good enough and the friction is less.
Friction is defined for a pair of surfaces. For example, the friction between iron and steel surface is different than the friction between iron and wood surface. Friction of only iron surface is meaningless.