Question

what is the basic reason why consumers,industrialists etc.
should make choices?

Answer 1

constrained optimization

Answer 2

Basically because of scarcity- we live in a world where most often than not, resources, be they in the form of money (capital), land, labor etc, are constrained and everybody must make decisions accordingly. Sometimes, the resources are in abundant supply and we must make appropriate decisions too so that we can maximize on these, for instance, when there is excessive labor supply coz of unemployment

Answer 3

As they say "no one can have it all" because we live in a world deemed to be plagued by scarcity ,which is mother of all the reasons (both basic and complex ).