Why? because a dog is usually what someone would keep as a pet not eat so it can feel sad if you see an animal commonly use for companionship being used as food
Why not feel sad when lambs, cows, chickens and pigs are killed as food. Humans are omnivore we can find sustenance from vegetables, milk, and grains, without the need for killing.
I can not accept it ethical to kill animals because they out of pure "estethical" reasons are killed, like for example rats. Rats are intelligent animals that is filling an important role as scavengers in our surroundings. Yes they, like a l l wild animals carry deceases like salmonella but they are scavengers and not for cuddleing.