The system which protects against pain is one which also gives us pleasure. During the birth process the baby itself releases its own endorphins, so that in the hour following birth, both mother and baby are impregnated with opiates. We all know that opiates create a state of dependency. Now we are in the position to understand that when mother and baby are close to each other and have not yet eliminated their opiates, the beginning of a dependency, or attachment is created. One can also understand that when sexual partners are close to each other and impregnated with opiates, another kind of dependency is created, which follows the exact same model as the attachment between mother and baby.
Lactation is also necessary for the survival of mammals.
It is not surprising that the reward system is involved. When a woman is breast-feeding, her level of beta endorphins peaks at twenty minutes. That is why after being breast-fed, babies sometimes act as if on a high. Our knowledge of endorphins is still new. Only 20 years ago, Pert and Snyder, in an historic article published in Science revealed the existence of cells sensitive to opiates in the nervous tissue of mammals.