We get attached to children and lovers with exposure due to oxytocin. Only when the natural switches for releasing it are shut off does exposure cease to have this effect.

I'm trying to relate this to your theory that men find it harder to kill their infants than women do. The influence of oxytocin discourages killing of those you are attached to and mothers get more of this than fathers if for no other reason than a crap load getting released during childbirth.