During the 1940s and 50s, an Austrian psychologist named Konrad Lorenz studied the behavioral patterns of geese.
One of the things he observed was the egg-retrieving behavior of the greylag goose. When an egg rolls out of a goose's nest, it gently uses its bill to roll it back in. However, when Lorenz took an egg from the nest and placed it next to a larger round white object, the goose preferentially rolled the larger object back into its nest while ignoring the real egg. He called this larger object a superstimulus. It was an abnormally strong stimulus that was able to hijack the bird's normal behavioral pattern in a maladaptive way.
Our brains are wired to respond to the stimuli with which they evolved. For example, our natural taste preferences tell us that fruit is good. But what happens when we concentrate that sugar tenfold? We get a superstimulus. Our brains are not designed to process that amount of stimulation constructively, and it often leads to a loss of control over the will, or addiction.
It's a very similar process to drug addiction. Addictive drugs are able to plug directly into the brain's pleasure centers, stimulating them beyond their usual bounds. Food superstimuli do this less directly, by working through the body's taste reward pathways. In fact, sweet liquids are so addictive, rats prefer them to intravenous cocaine. You can't take just one hit of crack, and you can't have just one Hershey's kiss.
Our bodies are finely honed to seek out healthy food, but only in the context of what we knew when our tastes developed during evolution. If all that's available is grass-fed meat, pastured eggs, vegetables, fruit, and nuts, your appetite will naturally guide you to a healthy diet.
If you surround yourself with superstimuli such as sugars, refined grains and MSG, your body will not guide you to a healthy diet. It will take you straight into a nutritional rut because it's not adapted to dealing with unnatural foods.
Your brain is pretty simple in some ways. It has these very basic hard-wired associations, like "sweet is good" and "free glutamate is good". If your brain likes a little bit of sweet, then it really likes a lot of sweet. If it likes a little bit of glutamate from meat, then it really likes a flood of glutamate from MSG. Just like the graylag goose that prefers the big white ball over her own egg, your brain drives you to ignore normal stimuli in favor of more potent superstimuli.
This explains the partially true saying "Everything that tastes good is bad for you". Why would your body deliberately encourage you to damage your health? In our hunter-gatherer state, it didn't. In this age of processed food, our technology has outstripped our ability to adapt.