I don't think it's an intellectual thing. i.e. even if you believe this or that doesn't mean you couldn't be saddened. I think it just happens naturally. You ask why do we value one life over another. As I said before, I see our ducks mourn if one of their group dies. They aren't mourning for the rat that was found dead in their coup. I think it's just the way it is. We have 4 ducks. One is lame but gets around very slowly. When it's time to go in at night (so the raccoons won't kill them) they all come in together. The ducks slow down so that the lame one wont be left alone. They are very social animals as are humans. Some humans are more social than others of course.
I can think one thing (there is no such thing as death- we never were, etc) but that is only part of me. It's intellectual almost. I guess the more deeply you believed that, to the bottom of your core, that perhaps you would be less emotional. Maybe you are more in that realm. Personally, I tend to be very affected by some people in my life. When they are gone, I grieve. Birth and Death are so big in our lives. I cried when my kids were born. I haven't really given an answer, just more of my rambling. :?