The possible reasons are many. Firstly, it could be that the dog was lost and wandered into the junkyard and then couldn't find its way out. Secondly, the junkyard is not a friendly place for a dog. There are no friendly humans to interact with, like in a normal neighborhood. And thirdly, the dog may have been born there from stray parents and has never known a different, better life. It has to endure the cold, rain, and heat all alone in this junkyard.
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.
The junkyard dog might be sad because it lives in a junkyard. It could be lonely, with only scraps and old cars around. Maybe it was once a pet but got abandoned there. It has to fend for itself, looking for food among the junk, and is often feared by people who pass by, without understanding its real situation.
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.
One possible reason could be the loss of a loved one, like in the case where the cat's owner died. Another might be separation from friends or family, as the cat that lost its mouse friend. Or neglect, such as when the family focused on the new baby instead of the cat.
One reason could be neglect. If the owner doesn't take proper care, like not feeding it enough or not providing shelter, the horse would be sad.
One reason could be the difference in their destinies. Silver is often tied to the future of his own world, while Blaze has her own duties in her dimension. This can lead to a sense of separation and sadness. For example, if Silver has to go back to his time to fix things and Blaze can't leave her world, it's a sad situation.
One possible reason is the pressure of always having to be perfect. As a virtual idol, Miku is expected to look and perform flawlessly all the time. Any glitch or mistake could be seen as a failure, which might make her sad.
Another reason could be unrequited love. As I mentioned before, if Meowth has feelings for another but is rejected, that's very painful. It can lead to a long - lasting sadness.
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.
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 possible ending could be that another kind player comes across the sad dog and decides to take it in as their own. The dog then gets a new home and is happy again.