Probably the lack of identity. They are just numbers in the Empire's military. No one really cares about who they are as individuals. For example, if a stormtrooper dies, they are quickly replaced, and no one mourns their loss.
Once there was a stormtrooper named TK - 421. He was always alone. His comrades didn't really care about him. He was just a number in the vast army. One day, during a battle, he got separated from his unit. He wandered in the desolate battlefield, hearing the blasts all around. He knew he was likely to die alone, forgotten by the Empire he served so loyally. It was a truly sad fate for a stormtrooper.
There might be a stormtrooper who had a family before being conscripted into the Imperial army. He missed his family terribly but was never allowed to go back or even communicate with them. He spent his days feeling lonely and trapped in a life he didn't really want, longing for the simple joys of his former life with his loved ones.
Losing contact with Earth could be really sad. Imagine an astronaut who suddenly can't communicate with their family or mission control. They would feel so alone and cut off from everything they know. It's like being lost in a huge, empty void.
The most heart - breaking part could be when the Furby is completely forgotten by its owner. After all the good times they had together, to be left alone in a corner or a box, it's really sad.
The loss of a loved one. Just like in the story where a mother was driving with her young daughter and a speeding car hit them. The mother saw her daughter die in her arms in the wreckage. That moment of losing someone so precious is the most heart - wrenching part.
Sure. There was a stormtrooper named TK - 421. He was assigned to a remote outpost. One day, he saw a group of rebels who were actually just trying to find food for their starving families. But he was ordered to open fire. He hesitated for a moment, and that moment of hesitation cost him. His superior officer shot him on the spot, deeming him a traitor. His last thoughts were of the injustice of it all.
When a penguin is bullied by others. Say there's a small, shy penguin who is made fun of by a group of mean penguins. They push him around, take his toys (like the sled or the fishing rod). The poor penguin would be so sad that he would hide in his igloo and not come out for days, feeling lonely and unwanted.
The saddest part is probably when the cat is abandoned. It's heartbreaking to see a cat that has known love and a home suddenly left all alone, not understanding why its humans are gone.
If a monster discovers that they don't fit in anywhere in the school. Let's say a hybrid monster, part - unicorn and part - troll. The unicorns don't accept him because of his troll side, and the trolls don't like him because of his unicorn features. He feels like he has no place to belong in the whole school, and this sense of isolation is extremely sad.
Maybe a stormtrooper gets lost during a battle and is found by a group of peaceful villagers. One of the villagers, a young woman, nurses him back to health. As he stays with them, he develops feelings for her. He then has to decide whether to stay with her or go back to his military life.