“ There is no human nature, since there is no god to conceive it. ”
When the rich wage war, it's the poor who die.
~ Jean-Paul Sartre
It answers the question that was tormenting you: my love, you are not 'one thing in my life' - not even the most important - because my life no longer belongs to me because...you are always me.
Man is condemned to be free; because once thrown into the world, he is responsible for everything he does. It is up to you to give [life] a meaning.
You are -- your life, and nothing else.