This column by Judith Lothian of Lamaze International provides a very nice overview of how the hormonal changes that take place during natural childbirth prepare both mother and baby for breastfeeding -- and for the bonding that accompanies it.
She writes that it's not only a baby being born; the laboring woman also is being born as its mother.
While the hormonal orchestration of labor and birth sets the stage in a more immediate way for the process of breastfeeding for both the mother and her baby, she writes,
The disruption of the hormonal orchestration of labor results in women giving birth with relatively low levels of naturally occurring oxytocin, endorphins, and catecholamines. Consequently, the outcome of low hormonal levels is a less responsive mother and baby.