first, there is the sensation of eating the food. This includes what it tastes like, What it smelles like, and how it feels in your mouth. this last quality - known as "orosensation"- can be particularly important. food companies will spend millions of dollars to discover the most satisfying level of crunch in a potato chip. their scientists will test for the perfect amount of fizzle in a soda. These factors all combine to create the sensation that your brain associates with a particular food or drink.
the second factor is the actual macronutrient makeup of the food - the blend of proteins,fats, and carbohydrates that it contain. in the case of junk food,food manufacturers are looking for a perfect combination of salf,sugar,and fat that excites your brain and gets you coming back for more.