Chocolate -- It's mostly thanks to the sugar and fats, but also the anandamides (chemicals that cause a similar effect to the THC in marijuana), phenylethylamine and tyramine (which cause effects similar to amphetamines), caffeine, and theobromine (a stimulant similar to caffeine). Lots of stimulation makes you feel happy post-chocolate, and you're addicted because you want that "happy" feel again! Pizza/Burgers -- The combination of high-sugar tomato sauce, fatty processed cheese, high-flour buns and crust, and greasy beef and pepperoni cause spikes in your blood sugar. The refined carbs hit your system quickly, and that feeling is highly addictive to the human body. [caption id="" align="aligncenter" width="580"] Source: authoritynutrition.com[/caption] Fries/Chips -- Fat (alone, and when combined with the sugar in ketchup) causes your body to produce more dopamine, the hormone responsible for pleasure. High fat foods cause you to feel better, and thus you become addicted to that feeling. Desserts -- Once again, sugar and fat play a huge role in your addiction to cake, brownies, ice cream, cookies, and pastries. Sugary foods cause your body to produce more insulin, which in turn makes it easier for your body to turn tryptophan into serotonin--the "feel good" chemical. In essence, sweets = feel good. Highly addictive! Diet Soda -- The sweetness of the soda tricks your brain into thinking that your body is getting sugar, but when there is no sugar to be had, your brain gets demanding and sends signals saying, "Give me sugar!" Drinking more diet soda won't solve the problem, and thus you end up drinking A LOT more artificial soda.