The real reason we have trouble getting our girlfriends out of bed in the morning.
From the very beginning of human history, mankind has been obsessed with dongs. To our earliest ancestors, dongs weren't just inconvenient fistfuls of flesh that got in the way when wrestling saber tooth tigers. Instead, dongs were seen as powerful and mysterious entities. Dongs could bring rain to weary crops. Dongs could cure stubborn illnesses. Dongs could even make a chick's boobs get bigger and start squirting milk.
Unsurprisingly, early man's obsession with dongs went a little further than ours does today. Instead of just walking around talking about awesome dongs are, they demonstrated their love for dongs by creating cultures and religions that celebrated and worshipped dongs. They carved giant sculptures to show how awesome their dongs looked, they told stories about dragons and elusive white whales to metaphorically describe how big and powerful their dongs were, and they even invented long, swollen-headed weapons like arrows and spears so they could kill shit with their dongs, at least in some figurative sense.
At the same time, womankind has also been perpetually obsessed with dongs, but with one potentially notable difference. Whereas men have spent their time talking about how awesome dongs are, women have probably spent their time complaining about how much more awesome dongs could be. The historical evidence for this theory lays in the fact that in almost every major culture over the past 30,000 years, men have by-and-large banned womenkind from their dong-related activities (a trend still observable in many of today's surviving primitive cultures) and women have, in turn, owned dildos.