why excessive dieting will reduce your sex drive

Caloric restriction in almost any vertebrate species will reduce levels of the reproductive hormones.

Many people assume the mechanism is "stress" -> high cortisol -> shutdown of reproductive hormones. That chain of events in fact does happen with lots of other types of stress, but it's now turning out that caloric restriction is a unique kind of stress that does not always cause high cortisol and that involves a whole bunch of other hormones. The more likely mechanism in this case is falling leptin. Leptin is a hormone put out by fat tissue that can be considered sort of the body's "fat report", and it directly affects the hypothalamus. Basically high leptin (= high body fat) tells the hypothalamus "Go ahead with breeding, you have plenty of fuel reserves" and low leptin tells the hypothalamus "Stop breeding, there's some kind of fuel crisis happening." Dieting causes an unusually extreme drop in leptin, and the hypothalamus responds by shutting down the reproductive hormones.
Here's a relevant study (small n but otherwise a good study http://www.ncbi.nlm.nih.gov/pmc/articles/PMC154448/) from 2003, on hormone values in eight lean men who were put on a sharp caloric restriction for four days. Leptin immediately plummeted to 10% of initial values, far out of proportion to the actual fat loss that was happening. Then all the reproductive hormones also shut down - testosterone dropped 40% and other reproductive hormones showed similar drops. But when men were given injections of leptin during a 4 day diet, all the reproductive hormones stayed at normal levels despite the dieting, indicating it is most likely leptin that is responsible for the changes in testo. Similar phenomena are seen in rats and in other mammals.
The above is an oversimplification but that's basically the idea. There are also effects on thyroid hormones & on growth hormone, both of which also partially shut down, and there are several other fuel-store-signaling hormones that are involved. See here for a more recent review. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2634963/




tl;dr - Severe diets cause an extreme plunge in leptin, which shuts down the reproductive hormones as an emergency response to starvation.