We, humans, are separated from all other animals by one distinct individual state.
That is, each of us can contemplate our own inevitable deaths, and even commit suicide because of or in spite of it.
If you really want to know what I believe it is? The answer is we are self-aware of choice...though many humans get to a point in their lives where they actually choose to feign ignorance and that leaves us with the world we now live in. Sad to say, but meh, that's life.