Dogs may be crying tears of happiness when their eyes well up upon being reunited with their humans, according to new research. The study at Azabu University's Laboratory of Human-Animal Interaction and Reciprocity is the first time that canine tears have been conclusively linked with emotion. Why would dogs cry tears of joy though? According to the scientific paper, "human participants rated their impressions on photos of dogs with or without artificial tears and they assigned more positive scores to the photos with artificial tears. These results suggest that emotion-elicited tears can facilitate human–dog emotional connections." From CNN:
"We found that dogs shed tears associated with positive emotions," Kikusui, who coauthored the research that published Monday in the journal Current Biology, said in a news release.
"We also made the discovery of oxytocin as a possible mechanism underlying it," [lead researcher Takefumi] Kikusui said, referring to the hormone that in humans is sometimes called the love or maternal hormone[…]
Humans often cry in response to negative emotions, but researchers didn't test to see if dogs did the same, too. They also don't know if a dog's ability to tear up plays a social function in the canine world.