Unknown Tear

A young boy cannot get around the fact that his dog is not really happy. He tries to make the dog happier by giving food to him and playing fetch. However eventually he does not succeed. After a while, the boy is alone reading a book until he suddenly starts crying, his mood deteriorates and he feels sorrow but the cause of that is unknown. Ultimately, the dog comes in and offers to the boy company showing that that was what the dog needed too after all.