Most often, tears in the eyes of dogs appear when they are left alone. This behavior is often seen in puppies at night who have just arrived at their new home. Also the dog may cry when you come home. Scientists have established that this behavior is caused by the hormone oxytocin. When a dog meets its owner, it releases a "love hormone". Dogs also may cry when they hear high-pitched sounds. You probably know that dogs can react differently to sounds. So when loud noises can be heard in your home, you can look at your dog and see tears in his eyes. And more dogs cry when they are bored.  And the most important reason why your friend may cry is when he is in pain. So be attentive to your dog and he will be very grateful to you.