It's science. And while modern science has brought us some pretty cool lights, like those on the new Cherokee, all the most beautiful cars use simple, circular headlights. Take for example this Cobra. The design is timeless, the car is iconic and revered.
Take a look at this BMW 507. This may be the sexiest car BMW ever made. It has circular headlights. As does this 250 GTO
Here's a few more.
Feel free to add your own.