The hormone released during nursing and orgasms also bonds humans to dogs

WSJ: For all of you moms out there, oxytocin is one of the hormones released from 1. nursing and 2. orgasms. Kind of an I-feel-really-good-about-you hormone. 

The discovery concerns the mammalian hormone oxytocin, which evolved around 500 million years ago from an ancestral version still found in fish and amphibians. The hormone evolved to play a key role in what makes mammals mammalian.

Other newborn animals typically fend for themselves: Crocodiles, for example, are catching insects soon after birth. But mammals develop slowly, and mothers have to feed their newborns. Oxytocin evolved to make this possible, prompting mothers who are nursing to produce more milk as their babies demand it.

Evolving the means to nurse the young was only half the battle. You also have to want to take care of them and to invest zillions of calories in generating milk and fending off predators. And you need to be able to recognize your offspring in a crowd, so you don’t waste your energy helping others to leave behind copies of their genes.

Oxytocin helped to solve both problems. Around the time of birth, female mammals release oxytocin in some brain regions, and the hormone allows them to register and recall their offspring’s smell, appearance or voice. Oxytocin rewards such maternal behavior with feelings of well-being.

So, extend that to relationships with pets - apparently, although I've always read about it in the nursing/orgasm contexts, there's a male version, too, at least insofar as bonding with pets is concerned:

Reporting in the journal Science, Miho Nagasawa of Azabu University in Japan and colleagues observed that modern dogs and their owners secrete oxytocin when they interact with each other. Remarkably, dogs who gaze the most at their humans during interactions had the biggest oxytocin rise—as did their humans.

The scientists then spritzed oxytocin (or saline, as a control treatment) up the dogs’ noses. The oxytocin caused female dogs to gaze more at their humans…whose own oxytocin levels rose as a result. All of this only affected dogs and their owners. Hand-reared wolves and their owners didn’t react in the same way to the treatment, and dogs administered oxytocin didn’t gaze any longer at humans who weren’t familiar to them. In other words, dog and human brains seem to have evolved at lightning speed to co-opt oxytocin for bonding between our species.

This sure helps to explain people who use baby talk with their dogs.

Read the whole article in the WSJ. H/T GeekPress.