According to a recent study, the difference between people who crave chocolate and those who don't, is that the cravers are host to bacteria that "need" that chocolate. Everybody has a very large and diverse population of bacteria living in their digestive tract. The composition of these populations varies from person to person.
A recent British study found that half of people that suffer from depression crave chocolate and feel better when they eat it. Is that just the bacteria messing with their heads? I think the scientists have some work to do here.