It seems that there's a good reason why dogs are always seen as man's best friend. Scientists have found that dogs are the only animals that can read emotion in faces much like humans. The finding suggests that like an understanding friend, dogs can see if we are happy, sad, pleased or angry. When humans look at a new face, their eyes usually look across the left, falling on the right hand side of the person's face first. A possible reason for this is that the right side of the human face is better at expressing emotions. Scientists have now shown that pet dogs also have "left gaze bias ", but only when looking at human faces. No other animal has been known to do like this before. Dr. Kun Guo showed 17 dogs pictures of human, dog and monkey faces as well as something else with his team. The dogs' eyes and heads show a strong left gaze bias when the animals see human faces. But this did not happen when they were shown other pictures, including those of dogs. Guo suggests that over thousands of years living with humans, dogs may have developed the left gaze bias as a way to guess our emotions. "Recent studies show that the right side of our faces can express emotions better than the left. If  true, then it makes sense for dogs and humans to see the right hand side of a face first." Why are dogs seen as man's best friend according to the passage?
Because dogs are the closest animals to humans. Because dogs can read the emotions in humans' faces easily. Because dogs can understand us better when we feel sad. Because dogs are good at expressing emotions.