This discussion immediately reminded me of a few of George Carlin's comedy routines, particularly the one where he addressed our concept of "the sanctity of life". Carlin's assertion was that said "sanctity", which we all know to be very selective (meaning that we as humans get to choose which forms of life are sacred and which are alright to terminate), is rooted in our own vested interest in self-preservation as living human beings and our ego-driven desire to feel noble. In ideal circumstances, no living organism desires to die-- and humans are, like others, driven by a powerful survival instinct. Of course the consensus among most of human society is going to reflect the urge for self-preservation. It's not because we're any better than any other organism on the planet. We just happen to be one of the few (if not the only) that is actually conscious enough of our own mortality to emote over it.
The theory I've heard most often, for why the human race as an entity behaves so self-important, blames our actions on arrogance. However, I tend to believe that, beneath the arrogance, at the root of all our posturing, mankind is ruled by the fear and insecurity that comes in knowing that the shadow of death is hanging over all of our heads, and that everyone, regardless of individual identity, has a finite time on Earth and an expiration date. Cognition of mortality is a massive burden, particularly when it is also hardwired into us to carry out our animalistic duty of self-preservation to the best of our abilities. The very reason "the sanctity of life" exists in the first place, and why it is selective, is to give us the sense that our lives are somehow protected by right of simply existing, and that we also have the natural right to destroy any other form of life that might threaten our continued existence. Of course, when we look at what our science has taught us about the world around us, we know that as a species our presence is a single drop in the ocean compared to the existence of the Universe. It's not at all irrational to say that, should the Universe as we know it ever cease to exist, it is a good bet that humanity will have ultimately been nothing more than a small footnote in its long history. And the Universe is cold, unfeeling, and completely impartial to whether our species survives another 5,000 years or another 5 days. As a whole, I think humanity is terrified of that. We're alive right now-- we want desperately to believe there's some logic to us being here, and that we're important. Personally, I don't like nihilism because I find it really depressing-- but I can't help thinking there's a grain of truth in the idea that, someday several trillions of years from now, when nothing is left of us, nothing we will have ever accomplished within our entire span of existence will have really mattered that much, no matter how much importance we may have attached to certain things. Not exactly the most pleasant or uplifting of thoughts. So it's no wonder we act like human life has some degree (although, again, it's still selective based on the circumstances) of sanctity.