Our caveman ancestors had an inbuilt urge to eat foods with a sugary taste, as sugar is a ready source of energy which was valuable to their survival. Eating sugar made them feel pleasure by stimulating the release of a chemical called serotonin, and probably other mood-enhancing substances as well.
This survival instinct is still part of our genetic makeup today. The trouble is that these days, we eat far more sugar than our ancestors ever could. They could only eat sugar sporadically, in the form of honey, berries and other fruits and roots when they came across them. Not only are these sources of sugar constantly available to us, but we also have highly refined and concentrated forms of them which give us a sugar 'hit' that our ancestors would never have got.
When we eat large quantities of sugar today (and other carbohydrate foods such as flour, bread, rice, potatoes and fruit which break down into sugar in the process of digestion), our blood sugar shoots up high. This triggers our pancreas to release far too much insulin. (Insulin is the hormone which controls whether the sugar is burned off as energy or turned into fat for storage in our fat cells.)
The sugar hit and the exaggerated insulin response makes our blood sugar level go too low. This signals to our brain that we need to replenish the blood sugar, and so we feel hungry again. In this way, our blood sugar/insulin control mechanism becomes imbalanced.
This imbalance not only makes us feel hungry, it causes us to lose the effect of the 'feel-good' chemical serotonin, and possibly other hormones such as leptin which tell us when we are full. So we eat more in an attempt to both feel better and to stop our hunger, and so the cycle goes on.
This is how sugar and carbohydrate cravings begin, and the only way to stop them is to cut down or eliminate the sugar and other refined carbohydrates from our diet. A low GI diet may help, but for many people who have an exaggerated response to carbohydrates of any type, the best solution is a very low carb diet.