Humans (and many other, but not all, species) appear to have the innate capacity for attachment and bonding. The development of human attributes such as empathy, intimacy and what we call the "conscience" are dependent on what we call attachment and bonding.   This relates to humans being an interdependent social species.

We know that infants who are not held and cuddled and soothed, who do not experience 'mirroring' of facial expressions, touch, sound, are more likely to exhibit behaviors that fit the diagnostic criteria that we call Reactive Attachment Disorder, than are children who do have the aforementioned experiences. Other conditions, such as autism, also effect attachment and bonding.

"This American Life" did a show that illustrates Attachment issues.