John Bowlby created Attachment Theory in 1940:

Infants become attached to adults who are sensitive and responsive in social interactions with them, and who remain as consistent caregivers for some months during the period from about six months to two years of age. When an infant begins to crawl and walk they begin to use attachment figures (familiar people) as a secure base to explore from and return to.

Parental responses lead to the development of patterns of attachment; these, in turn, lead to internal working models which will guide the individual's perceptions, emotions, thoughts and expectations in later relationships.

Separation anxiety or grief following the loss of an attachment figure is considered to be a normal and adaptive response for an attached infant. These behaviors may have evolved because they increase the probability of survival of the child.