I'm pretty sure personally that it's built into our biology. In the words of Charlie Brooker:
"Imagine! Nothing to kill or die for! And no religion too! It's amazing, literally no one's ever had that thought before.
Okay, so there's always the possibility that the same part of the brain that handles fuzzy spiritual feelings is the same part that handles love and sorrow and pity and joy; the same part that makes us create songs and jokes and books and art and brightly coloured computer games in which an animated weasel collects starfish in a fountain; so once we wipe it out we might all be left scampering around the planet like thick, bipedal, cultureless mice - rutting, foraging, scratching behind our ears and doing very little else. But look on the bright side. No more religious conflict AND no more novelty ringtones. Two almighty evils erased for the price of one. Bargain."