In the suburban district of a city lived an old woman quite alone. Her husband died thirty years ago and two years later, her only son went away with lung cancer. So she had no family still living and her only friend was a little white dog who went everywhere with her - with one exception. The dog loved the fireplace in winter, and after the old woman went to bed he would sometimes go and lie in front of the warm coals. Usually though, the dog lay on a rug right next to the bed. Deeply as the old woman loved her dog, she wouldn't allow the dog on the bed with her. However, if she became frightened or had a nightmare, she would put her hand down to her little white dog and he would lick it to calm her down. One night she was reading the newspaper just before going to sleep. She trembled and pulled the duvet up around her as she read that a mental patient had wandered out from a nearby hospital. No one knew if the patient was dangerous or not, but he was a suspect in the murders of several women who lived alone. The woman turned off the lights and tried to sleep, but she was frightened, and turned over continually. Finally, she reached down to where her little white dog slept. Sure enough, a warm, wet tongue began to lick her hand. The woman felt reassured and safe, and left her hand hanging off the side of the bed. As she turned to settle in comfortably she opened her eyes for a moment and looked through the open door into the living room. There in front of the fireplace, sat her little white dog, gazing at the coals and wagging his tail. And down beside her bed, something was still licking her hand. Why didn't she allow the dog on the bed with her?
Because the dog was too dirty. Because she was fearful of dogs. Because the author dislikes keeping dogs. The author didn't mention the reason for it.