Hope

For starters, I believe that hope is something humans have that nothing else does.  Of course, there's no real way to prove this for sure, but I think I ought to make the point of hope anyway.  Normally I'd say that love is what makes us human, but quite frankly I think many other animals possess it.  Hope, however, seems to me to be unique to people.  Hope has an incredible power to pull us back from the brink of emotional breakdown.  Even when we feel like we have nothing else, there is usually (not always, but usually) still a subconscious feeling that everything is going to get better.  Perhaps it's not hope, exactly, but our resolve to be able to hope in even the toughest of times.  I don't know if I have the right words to explain it properly, but I believe we are human because we can endure and even hope.  Hope is a trait that, if left without, humans would be rendered unable to go on.