I'm not hungry, but right now I'm thinking about driving to Trader Joe's to buy a bag of their plantain chips. The last time I bought them, I ate the entire bag in one sitting. I wasn't hungry then, either. But that salty, slightly sweet, slightly greasy crunch? I just can't get enough. It calls to me.

I'm not alone, either. Many of us find ourselves eating not simply to sate hunger, but to scratch a sort of itch deep in the brain, an urge to ingest something particularly delicious. A strong pleasurable response to food makes plenty of evolutionary sense; it's natural that we get pleasure from that which sustains us. But why are some things, like chocolate for most people, or plantain chips for me, so irresistible? Is our obesity epidemic rooted somewhere deep in the brain, buried in that pleasure center?

Consider a hunter-gatherer in the Stone Age, looking for adequate nourishment to sustain herself and her family through a rough, dry season. The foods she prized most would be high in caloric density (general energy); fat, starch, and sugar (all high-energy nutrients); salt (an important mineral for health); as well as particular flavors and textures (such as umami or creaminess) that signaled good food based on historical precedent. For example, she might know that a sweet berry is safe to eat, while a bitter berry is not. Foods high in these desirable qualities have come to be called palatable. Many of the highly processed foods we eat today—potato chips, candy bars, breakfast cereals, pizza—are extremely palatable, precisely because they offer many of the qualities that would have sustained our ancestors.

Researchers have found that consuming food rated highly palatable causes the brain to shut down the feeling of fullness and satiety. The more palatable the food, the less filling it is per calorie. The brain's reward system wants to reinforce the consumption of these high-energy, satisfying, and safe foods. Your brain's reward system is less concerned with how you look in your bikini, and more concerned with your survival.

The brain ensures that you acquire energy by producing opioids, pleasurable brain chemicals simulating the effect of opiate drugs, when you eat palatable foods. Many former alcoholics and drug addicts develop a powerful sweet tooth after becoming sober, in part because the chemicals released by the brain when it eats sugar are similar in effect (albeit milder) to the addictive substance they used to depend on. Kicking a sugar addiction can even lead to symptoms of withdrawal. This drive is known as hedonic hunger, pushing us to continue consuming the foods we crave, without ever satiating our hunger.

And what happens when the reward system is confronted with large amounts of easy-to-access, highly rewarding, and palatable foods? Contemporary processed foods are hyper-palatable, in part thanks to food engineers who expertly combine salt, fat, sugar, and certain flavors to create food that we literally can't get enough of.

Think of the lowly cookie, which began long ago as a simple, yummy mixture of butter, wheat flour, and honey that could make a medieval farmer smile after a long day of toil. Over the centuries, however, cookies have evolved, with each generation of chefs layering in new ingredients to make them ever more delicious. When Europeans discovered chocolate and cane sugar in the New World, we couldn’t resist adding them in to create chocolate chip cookies—something far more delectable (and intense) than before. Eventually food scientists were struck with a flash and added in salty peanut butter, to make them even more rich and mouth-watering. Now the culinary arms-race has produced not only fudge-covered, chocolate-chip, peanut butter cookies, but topped them off with bacon for the umami coup-de-grace. That’s something a hunter-gatherer could have lived off of for a week, and so the reward system of the brain will keep us coming back for more.

The incredible abundance of calorie-dense food we now have is a blessing. Few of us have to worry about starvation or malnutrition. Yet our brains didn't evolve in such abundance. The foods in our current environment hijack the reward center of our brain, doling out mental cookies to go along with the actual cookies. This is why “just eat less and exercise more” is so much harder than it sounds—you have to overcome your intense, hardwired desire for more of the rush you get from addictive, delicious, hyper-palatable food. With an abundance of inexpensive, highly-rewarding food at our fingertips, the increasing prevalence of obesity should come as a surprise to exactly no one. It's hard to overcome thousands of years of programming.

Does this mean that we should all switch to a diet of unflavored mush and celery sticks, and that plantain chips (and Doritos) should be banned or taxed into oblivion? Of course not. The intense pleasure we get from food, along with the flourishes and traditions that accompany it, is one of the great joys of life—something that most every human being shares. Understanding what goes on in the brain (as much as in the kitchen and in our tastebuds) helps us to address this universal part of being human, and to understand why we do what we do.

What foods do you find near-impossible to resist?