What makes the dog-human bond so powerful? In a word, reciprocity.
Japanese researchers Nagasawa, Kikusui, Onaka, and Ohta found that dogs and humans have a give-and-take relationship so powerful that it's controlled by the very same hormone—oxytocin—that binds mothers and infants, reduces anxiety and depression, and builds trust and intimacy.1
Scientists speculate that oxytocin played a role in the domestication of dogs, leading us to share many of the same social signals we use with our own children. We're actually biologically programmed to love each other!
1 Nagasawa M, Kikusui T, Onaka T, Ohta M. Dog's gaze at its owner increases owner's urinary oxytocin during social interaction. Hormones and Behavior. 2009;55:434-441.