Its Badass Origins
Have you ever looked a gorilla in the eye? If so, you're either lucky to be alive, or currently being dismembered by a primate. Gorillas hate that shit. Eye contact and smiles are like subtle ways to tell a gorilla you'd like to see them try to kick your ass. And it's not just normal animal skittishness. Gorillas have a serious problem, and it's your stupid face. One zoo even invented special glasses to protect visitors from eye contact -- instead opting to make making look like they're having the most prolonged orgasms ever ...
As mankind made its evolutionary transition from ape to human, scientists think that there was a time when a flushed cheek and a downcast eye was a key survival instinct. When one species was still evolving into the other, it meant your genetic line wouldn't end in a dismembered pile of limbs. Blushing would have been especially useful, since it's an involuntary reaction. No matter how hard you tried to maintain eye contact, your blushing face broadcasts just how close you were to soiling your loincloth.