The combination of sugar, salt, and fat, has been shown to create a dopamine spike in the brain, similar to drugs like heroin and tobacco, which is why obese people are typically hooked on these kinds of foods. source
It is also a good explanation for why a lot people who quit smoking or doing drugs typically start eating a lot or gaining weight. Substituting one dopamine rush for the next.
There Was actually a lawsuit against McDonalds a while back alleging that they knowingly used addictive ingredients to make people addicted to their food. The addictive ingredients were sugar, salt, and fat. The lawsuit was dismissed.