Part of what I described in my post would explain this. As far as we know, animals get some enjoyment out of basic behaviors, but they don't produce meaning in life for themselves. There are sizeable differences in how humans and animals experience the world, and that translates into how we treat them differently. Taking an animal's life does not constitute harm like taking a person's life, because we aren't taking the same thing away from an animal as we are taking away from a person.