The dog might be trained to do that as part of a performance or just because it learned that behavior gets a reaction from people around it.
Maybe it's hungry or wants something from its owner.
Maybe it's for a fun adventure or to catch someone's attention.
Maybe it's waiting for someone to give it permission to eat the treat. Or it could just be enjoying the moment of having a treat nearby.
Maybe it's alert or waiting for something. Dogs often sit up when they're paying attention.
Maybe it lost its favorite toy or was scolded by its owner. That could make it sad and just sit there.
Maybe it's tired and needs a rest.
It could be that the dog is in some sort of magical or fantastical situation where fire doesn't harm it. Or, it could be a mistake and the dog is supposed to get out of there quickly!
Well, it could be that the dog is the king of the cartoon world and the throne is its rightful place. Or maybe it's a dream sequence or a magical occurrence. Who knows? The possibilities are endless in the world of cartoons.
The sad dog could be sitting because it lost its toy or didn't get the attention it wanted. Cartoons often show emotions this way to tell a story.