One possible reason could be that it faced some dangerous creatures in the time they traveled to. For example, if they went to a prehistoric time, dinosaurs or other large predators might have attacked the dog.
There was a girl who found a magical object that allowed her to time travel. She always brought her dog along. One day, they traveled to a future world where there was some sort of environmental disaster. The air was toxic. The dog was not able to adapt to the harsh conditions. Despite the girl's efforts to save it, the dog passed away. She learned a hard lesson about the importance of taking care of the environment from this tragic event.
One reason could be that the dog is feeling threatened. For example, if the man accidentally steps into the dog's territory, like its yard, the dog may bite to protect its space.
Illness could also be a factor. Just like in real life, if the dog in The Sims gets sick and there's no proper treatment or care, it can lead to a sad story. The dog would be weak and suffering, and its quality of life would deteriorate.
Magic is a common reason. Just like in the stories where a magical spell or potion causes the transformation. For example, a wizard might cast a spell on the boy as a punishment or by accident.
One reason could be abandonment. Maybe its owner didn't want it anymore and left it in the junkyard. Another is lack of proper care. In the junkyard, there's no one to give it regular meals or medical attention. Also, the environment is harsh. The dog has to deal with all kinds of dangerous and dirty things.
One possible reason could be self - defense. If the dog was attacking him and he had no other means to fight it off, he might have bitten the dog.
Another possibility is a curse from the supernatural world. Maybe he entered a haunted place or broke an ancient taboo, and as a result, a curse was placed on him to transform into a dog as a form of penance or to teach him a lesson about respecting the unknown.
It might be that the man made a deal with a witch and broke the terms, causing the transformation. Or it could be a symbol of his inner transformation and represented metaphorically as a physical change into a dog. There could also be a virus or a strange potion involved.
One possible reason could be loyalty. Dogs are very loyal animals, and they might watch over their owner while sleeping to protect them from any potential danger.
Magic or spells. In many fictional worlds, magic is a powerful force that can cause all kinds of transformations. If a magician casts the wrong spell or if there's some sort of magical accident, a man could turn into a dog.