A study published in the journal Current Biology found that a dog’s tear production increases with emotion.

The study measured tear volume in dogs that were reunited with their owners vs with unfamiliar people. Tear volume increased during the dog-owner reunions.

The researchers theorized that the tears were caused by an increase in oxytocin, a hormone that triggers feelings of love and connection in mammals.

They tested this theory on dogs by applying an oxytocin solution to their eyes and found a corresponding increase in tear production.

Finally, they showed people pictures of dogs with both teary and normal eyes, and—no surprise—we feel warmer emotions toward the teary-eyed dogs.

The researchers note that the close bond between dogs and humans is facilitated by these emotion-induced tears.

You can watch a video that summarizes the study HERE.