Empathy is that thing which makes suffering exist in your mind when it exists in others. And even among the non-sentient animals, it can extend to other species. The reason why we have to take it further than they do is because we have the mental capacity to construct a self-consistent system of ethics out of it.
We have to take it as far as emphathy combined with reason (which extends our ability to empathize) will take us, precisely because this is the only self-consistent approach other than sociopathy. If you're advocating the latter, then I'll admit that it is also fully self-consistent, and rejecting it is really just an ethical axiom that I subscribe to.
