Part II: "The Runaway Process"

Let's assume ancestral people started out plain-looking. Then through some minor genetic-shift, females developed a hankering for say, males with large heads. If this big-headed phenomenon happened to occur in bigger, better males, then females who chose them would probably produce more, and healthier offspring. Big-headed genes would proliferate in males, and the preference for them in females. This is all well and good, but sooner or later, conventional knowledge is that females soon come to focus single-mindedly on this trait, regardless of it's correlation with the overall quality of the male anymore.
Thus begins the cycle of runaway one-upmanship: over hundreds and thousands of generations the human head continues to get larger until no self-respecting male can get a date unless he's got that mega-dome. Natural selection steps back in to stop this nonsensical runaway process at the point when the quest for better sexual ornamentation is slowing the males down enough and attracting enough predators to kill them. To a degree, we have lost much of this natural system of checks-and-balances in the modern human world, so the nonsensical runaway system has free reign over the dominion of female sexual preference. This is where everything starts to turn terrible in human world.
In the natural world where the balancing factors are more available, the system proceeds in an orderly fashion. The arctic fox with the "Eat Me" sign wouldn't make it through a single winter with that bullseye on him unless he was very good at out outrunning polar bears. It is this underlying, and necessary consequence of his bold, and risky male behavior that the females reproductive system is attracting her to, often unbeknownst to her.