There's something creepy about a mother lacking enough oxytocin to nurse their own child, considering the emotions and mental state that such a thing would suggest.

Nonetheless, it's probably really common and normal, and likely serves as a vital check for the prevailing health of the population.

Which we now override by artificial means, because we can, and because the expectations for a social facade of parenting place a great degree of pressure to perform activities perceived as normal by external observers, as evidenced by my own comment.