Sugar "addiction" is an evolutionary trait. When mankind was hunter-gatherers, sugar was harder to come by. Our bodies react to sugar in a way to make us binge on it when we find it. Our bodies then store it as fat. That fat is burned when we go through periods of fasting, like when early humans went days or weeks without finding much food. It has also been suggested that some bacteria in our intestines release chemicals that cause sugar cravings.