I find the lock and key mechanism to be a little bit more intuitive. A key (ligand) binds to the lock (protein/receptor) and a conformational change (unlocking) occurs.

For those other people who may also need ultra simplification.

It's possible that he's isolated not just socially but also from dependence on civilization which creates physical demand for him to procure his own food and to handle the elements.

I think social isolation that's pathologic and leads to decreased activity is worsened by the convenience of civilization because then it becomes easier to avoid activity while also amassing caloric intake.