The science of the munchies
http://www.smithsonianmag.com/science-nature/scientific-explanation-how-marijuana-causes-munchies-180949660/
A team of European neuroscientists led by Giovanni Marsicano of the University of Bordeaux has found that, in mice, THC fits into receptors in the brain's olfactory bulb, significantly increasing the animals' ability to smell food and leading them to eat more of it. A big part of the reason why you might eat more food after using marijuana, the research indicates, is simply that you can smell and taste it more acutely.
This effect of THC has to do with the underlying reason why the chemical affects the human brain so potently in the first place. Likely produced by the marijuana plant as a self-defense against herbivores who might feel disorientated after eating the plant and avoid it in the future, THC fits into receptors that are part of the brain's natural endocannabinoid system, which helps to control emotions, memory, pain sensitivity and appetite. Our brains typically produce their own chemicals (called cannabinoids) that fit into these same receptors, so by mimicking their activity, THC can artificially alter the same factors in dramatic ways.
Because scent and taste are so closely related, it (THC) likely allows us to better taste flavors as well.
(The article also notes that our own bodies increase the level of endocannabinoids in the olfactory region when we fast.)
This new finding is likely just a piece of the THC-and-appetite puzzle. Previous research has found that the drug also acts on receptors in a region of the brain called the nucleus accumbens, increasing the release of the neurotransmitter dopamineand the sensation of pleasurethat comes as a result of eating while high. Other work has found that THC additionally interacts with the same sorts of receptors in the hypothalamus, leading to release of the hormone ghrelin, which stimulates hunger.