How do you explain the bond of human and dog?

this baffles me. the bond between a person and their dog is so strong, even if the dog is mistreated.. which is even more amazing. dogs gives unconditional love. humans are horrible, were violent, we steal, we do everything we shouldnt. yet dogs, commit no crime, and they are "mans best friend". i rescued y dog from a shelter almost four years ago.. she was horribly abused and neglected.. and yet she still is able to trust. she has become my shadow, theres not one thing i do w/o her with me.. shes the best thing in my life. but when other people come around she protects me with everything she has. the bond is so incredibly strong.. how is this so? its amazing how much a dog bonds with their owner.