I'm just a vehicle to my genes which they use to travel to the future. They only can travel when they have a suitable host who can reproduce them and can make sure that the next version has the best chances to reproduce again. So over time natural selection optimized for the vehicles to want to reproduce. Everyone who didn't have the desire to have kids in the past has none (other than rape victims) to move their genes forward.
Anyway this is a extreme oversimplification of the whole idea of The Selfish Gene