"Because bacon is one- to two-thirds fat and also has lots of protein, it speaks to our evolutionary quest for calories, Lundstrom says. And since 90 percent of what we taste is really odor, bacon's aggressive smell delivers a powerful hit to our sense of how good it will taste."
Of course since this article is from NPR, they don't make the obvious connection: it's not about calories, it's about nutrition. Your body needs the nutrients in animal fats and proteins to be healthy. And your body knows this, even if your brain has some foolish notions about vegetarianism. Hence, a craving for bacon.