Microscopic concept of friction:

No surface is perfectly smooth. A surface that appears smooth has pits and bumps that can be seen under a microscope. The figure shows two wooden blocks with their polished surfaces in contact.

A magnified view of two smooth surfaces in contact shows the gaps and contacts between them. The contact points between the two surfaces form a sort of cold welds. These cold welds resist the surfaces from sliding over each other. Adding weight over the upper block increases the force pressing the surfaces together and hence, increases the resistance. Thus, the greater the pressing force greater will be the friction between the sliding surfaces.

Tidbits

Pushing the opposite walls by palms and feet increases friction. This enables the boy to move up the walls