I have gone through these type of cravings. Nothing like eating some pizza on a bad day. Grains release exorphins in to the blood and then to the brain that actually do make you feel better. It is similar to a drug addiction. This realization was part of the process in me better understanding my messed up relationship with food.

Food is not a drug and should not be used in that way.

First link I could easily find, would like to see more:

http://www.nutramed.com/eatingdisorders/addictivefoods.htm

It's the same reason us PMS (term used loosely) ladies want something salty or something sweet at that time. As Scott said, there's certain non-nutrients in the food that our brain wants. It was mentioned on another question too, but I feel the same way when I'm sitting in class or studying. I just want to eat something, just to keep me/my mouth busy. Food is addicting!

Carbohydrate ingestion, blood glucose and mood. http://www.ncbi.nlm.nih.gov/pubmed/12034132

A series of studies have reported that a high carbohydrate meal, or diets high in carbohydrate, were associated with feeling less energetic. However, after a drink containing pure sugar most studies report no effect. Meals almost exclusively carbohydrate increase the availability of tryptophan and hence serotonin synthesis in the brain, however, a small amount of protein blocks this mechanism making it an uncommon response. In many individuals, poor mood stimulates the eating of palatable high carbohydrate/high fat foods that stimulate the release of endorphins. There is a tendency for those with lower blood glucose, when performing cognitively demanding tasks, to report poorer mood. In a range of situations an association between a tendency for blood glucose levels to fall rapidly, and irritability, has been found. Differences in the ability to control blood glucose levels influence the association between carbohydrate intake and mood. There is a need in future research to contrast the impact of carbohydrate on mood in those distinguished because of their pre-existing psychological and physiological functioning.

Endorphins are natural opioids made by your own brain that make you feel good. They are the reason that opioids from poppies have the effects they do. Eating foods high in carbohydrate (potato) or particularly foods high in both carbohydrate and fat (chocolate) is a particularly good way to releasing these endorphins. Endorphins are also released during exercise (runners high) to suppress pain. Feeling down makes you want to eat foods that make you feel better, this is more attractive than running a marathon.