Nature doesn't leave it to our powers of reasoning to figure out that ingesting fat and protein is conducive to our survival. Rather, it makes us hungry and gives us an intuitive sense that things like meat and fruit will satisfy our hunger. Nature doesn't leave it to us to figure out that fellow humans are more suitable mates than baboons. Instead, it endows us with a psychology that makes certain humans strike us as appealing sexual partners, and makes baboons seem frightfully unappealing in this regard. And, finally, Nature doesn't leave it to us to figure out that saving a drowning child is a good thing to do. Instead, it endows us with a powerful "moral sense" that compels us to engage in this sort of behavior (under the right circumstances). In short, when Nature needs to get a behavioral job done, it does it with intuition and emotion wherever it can. Thus, from an evolutionary point of view, it is no surprise that moral dispositions evolved, and it is no surprise that these dispositions are implemented emotionally.
-- Joshua D. Green, The Secret Joke of Kant's Soul