When two pretentious, arrogant idiots generate offspring.
The male douchebag and female douchebaguette will typically meet at a club and mate later on that night. If the conditions are just right, eventually the douchebaguette yields a douchebagel. Learning from the example of the parents, the douchebagel becomes an even bigger douche then its parents were, and thus the cycle begins again.
This is an unfortunate inevitability which sustains the world population of douchebags, forever subjecting the rest of us to their unrelenting douchebaggery.
Everyone on MTV's Jersey Shore is such a douchebag; I hope none of them get drunk enough to reprodouche.