There are compelling arguments, that are well-supported by research, suggesting that fast food is designed to be addictive, particularly through the loading of highly rewarding ingredients such as sugar, fat, and salt. These ingredients have the effect of making people -- and animals in lab experiments -- eat more and more of these foods.
However, addiction is a complex process, involving decision-making on the part of the individual, as well as individual differences in how desirable and rewarding the addictive substance or behavior is, and is affected by many other factors, such as where and when the activity takes place. Therefore, eating fast food won't automatically cause you to become addicted to it, and many people occasionally eat fast food with no ill effects.
Kessler, MD, D. The End of Overeating: Taking Control of the Insatiable American Appetite. Rodale: New York NY. 2009.