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!

References

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.