Dana Stevens at Slate suggests the mania is nostalgia. The series brings us right back to the drama of our own high-school relationships.
Bella is also the only person capable of brokering a truce in the age-old vampire/werewolf feud. And, as of this installment, a fresh contingent of supernatural meanies — an army of just-created vampires known as "newborns" — is being organized by a rogue lady vampire (Bryce Dallas Howard) for the express purpose of destroying Edward and Bella's love. This high-school romance actually has the world-historical significance that the rest of us only felt like our high-school romance had.
The Edward-Bella-Jacob triangle appeals to the ridiculous intensity of being a teenager, particularly a teenager in love. Maybe all falling for it says is we have yet to succumb to the overly cerebral grown-up world. And that's nothing to be embarrassed about!
Source: Flickr User HannahLeePritchett