They say the eyes are the window the the soul. Cliché, yes, but if that saying was to apply to one thing, it’s arguable there’s nothing more fitting than the stare a dog owner and his companion share. That’s because each time it happens, triggers in both brains are being fired, and the love hormone oxytocin is released, a new study has found.

Animal behaviorist Takeumi Kikusui of Azabu University in Japan and his team found evidence in their new study, published in the journal Science, that suggests dogs used their lovable stare to win over the hearts of humans thousands of years ago. When interacting and exchanging gazes, both dogs and their owners experienced rushes of oxytocin in their brains.

"Eye gaze from human to animals is usually threatening, not affiliative," Kikusui said, according to CBS News. "We speculated that some small population of ancestor of dogs show an affiliative eye gaze toward humans, due to the change in the temperament. In this process, we agree that there is a [possibility] that dogs cleverly and unknowingly utilize a natural system meant for bonding a parent with his or her child."

While previous studies have suggested oxytocin is active in the brains of dogs and their owners, the current study provides a more comprehensive look. In one experiment, the researchers looked at oxytocin levels in the urine of 30 human-dog pairs before and after spending a half-hour together. They found oxytocin levels increased among both dog and owner when the gaze was held for at least five minutes or for longer. Another experiment found that dogs given oxytocin were more likely to stare at their owners for longer, although this effect was only seen in female dogs.

By comparison, a control group of wolf-owner pairs turned up no changes in hormone production, despite the wolves being raised by the person staring into their eyes. “These results suggest that wolves do not use mutual gaze as a form of social communication with humans,” Kikusui said.

Speaking to Live Science, Kikusui said the findings may help uncover how dogs and humans came to be so close. He said it’s possible a small, naturally more friendly group of wolves may have come upon humans, creating a bond through their gaze. “We use eye gaze for affiliative communications, and are very much sensitive to eye contact,” Kikusui told Live Science. “Therefore, the dogs who can use eye gaze to the owner more efficiently would have more benefit from humans.”

Source: Nagasawa M, Mitsui S, En S, et al. Oxytocin-gaze positive loop and the coevolution of human-dog bonds. Science. 2015.