Hi @msgivings
I think that it is easy to say I will die for you, but I think you will never know if you will have the courage to do so, until you are in the situation, as you have said it is in our nature to survive.
The other point is also if you love each other so much that you would die for each other, one will stay behind and will be sad and morn the other one most probably for the rest of their live. Sad