When Robert Neville finds the sick puppy, he is already half mad with grief, depression, and loneliness. He pours his hope for companionship into taming the dog and making it comfortable with being close to a human. When he discovers it is sick and dying, his hope and energy go into trying to cure and help it recover. After the dog dies, Robert chooses to hold himself together and come to terms with the fact that he will die alone. As a consequence, he deadens his emotions.