Question

What is friction caused by? (theory)

Answer 1

The rougher the surfaces the more often “hills” will have to push over each other, and the steeper those hills will be. For most practical purposes friction is caused by the physical roughness of the surfaces involved. However, even if you make a surface perfectly smooth there's still some friction.

Answer 2

friction is due to the roughness of a surface.if the land is steep and good then we can bove a thing or push something without much force but if the land is bad we need more force.
The theory of the surface friction of solids which is developed is based on the assumptions that the adhesion theory of friction