I am not sure whether we can reduce this to psychological elements. After all, laboratory rats and wild animals that eat our food leftovers grow fatter too, not just us humans. Arguably, rat or raccoon psychology is rather different from ours.

I believe that the problem is not that different from the problem of alcoholism. Everyone lives in a society soaked with alcohol, some people do not feel an urge to drink at all, some people manage to keep their consumption in healthy limits for 50 years, some end up as hopeless wretches in 10 years.