Attachment behaviour has become a characteristic of many species during the course of their evolution because it contributes to the individual’s survival by keeping him in touch with his caregiver(s), thereby reducing the risk of his coming to harm
Bowlby grounds attachment behaviour in evolutionary biology, defining its function as proximity-maintenance for survival, and explicitly rejects the psychoanalytic equation of adult attachment behaviour with pathology or regression.
, Loss: Sadness and Depression (Attachment and Loss, Volume III), 1980thesis