Hollywood celebrities spend large amounts of dollars on it. The hunky stud at the local pub thinks he knows it. But the age-old secret has been carefully kept for millions of years. Yet, it seemed obvious to pre-mammalian reptiles that went so far as to evolve mouths full of beautifully crafted teeth. It's your beautifully bleached smile that makes you sexy!
Mammals, like us, have a set of dentition that are neatly divided into three distinct types of teeth -- the incisors at the front of your mouth, the molars in your cheeks, and the canines, that Dracula-type teeth that separates the molars from the incisors. The origin of this separation can be traced back to 300 million years ago, when our ancestors still looked like sprawling reptiles, the pre-mammalian therapsids.
These creatures, like the gorgonopsians (a group of therapsids), had long, sometimes sabre-like canines that was often interpreted as a deadly hunting device. However, there was a problem. Some herbivorous species that only grazed on plants, like the dicynodonts (herbivorous animals, varying in sizes from a rat to an ox, and like warthogs, had two tusks, that gave them their name, which means "two dog tooth").
So, if not for hunting, what were these impressive sets of pointy teeth for? Defence against predators? Nope! These prehistoric characters used them to seduce the beauties!
The rebuttal to this would be immediately after the gorgons went extinct, a therocephalian genus went through rapid convergent evolution to develop its own sabre teeth implying there was an empty ecological niche.