Death of a beloved character often makes a cartoon story sad. For example, in some stories, a cute puppy might die, leaving all the other characters and the viewers heartbroken.
Once there was a sad dog in a cartoon. The dog was left alone in an old house when his family moved away. He waited day after day for them to come back, sitting by the window with a forlorn look. His little tail that used to wag so happily now just hung limply. Every time he heard a car pass by, he hoped it was his family. But as days turned into weeks, his hope started to fade and he became more and more sad.
The movie "Up" has a very sad beginning. Carl and Ellie meet as children, both with a passion for adventure. They grow up, get married, and save up for their dream trip to Paradise Falls. But life keeps getting in the way, and Ellie becomes ill and passes away. Carl is left alone with his memories, which is a very touching and sad part of the otherwise adventurous story.
Loss is a common theme. For example, in "The Fox and the Hound", Tod and Copper's friendship is threatened by the fact that they are supposed to be enemies due to their species. They lose the innocence of their early friendship as they grow up, which is quite sad.
One cartoon sad story could be about a little kitten who got lost. It wandered the big city all alone, with rain pouring down on it. The kitten missed its warm home and its owner terribly. It meowed sadly as it searched for a familiar face or place, but everywhere was strange and cold.
It could be that the dog is on an adventure in the woods but has run into some trouble and is now sad. Or perhaps it's remembering a happy time with its owner and missing them while being in the woods.
The story of a little bird in a cartoon can be sad too. This bird was born with a broken wing. All the other birds could fly freely in the sky, but he could only watch from the ground. He tried so hard to take off but always failed. He was left behind in the nest most of the time, feeling left out and sad as he yearned for the freedom of flight.