It's a fact that sugar is addictive. Soda is a great example of sugar addiction.
Did You know that salt makes things taste sweeter? The fast food industry (as well as other industries) know this and "spike" their food to make people want more of it. Ever wonder why they say "you can't eat just 1 Pringle" (chip)? It's the combination of sugar and salt. They use in the right proportions to create an almost instant addiction.