The images below are from "Strength of materials" book by Timoshenko. As evident from the text, the author states that stress on the cross section pq can be resolved into normal stress and shear stress since the force P can be resolved into two components, one of which is parallel to the cross section pq, while the other one normal to pq. What confuses me is why are we resolving P at all? Why do we have a stress which is not in the direction of loading? What is the physical cause behind this stress?
I think there is something missing in my understanding of what causes shear stress while axial loading. So basically I want an explanation of what causes shear stress generally and in this specific case. (Equations, references and an intuitive explanation will help. Explanation in terms of molecular interactions will be much appreciated.)
TL;DR -> Okay, please provide explanation of what causes shear stress while axial loading.