The dog is the only one that can’t actually consent to space travel, and regardless, couldn’t possibly know the risks. It is innocent, and doesn’t deserve a violent fate.
Everyone else knows that they signed up to live in a metal box, with an artificial biosphere, which is all that separates them from the cold void of deep space. Also, said deep space is jam-packed full of things trying to actively break that metal box, if the crew doesn’t beat them to it first. And nobody knows that better than Seven.