but the basic premise is you exist because you exist, it's recursive to the point of nausea and you can't escape existing. even when you're dead you still exist because your matter still exists. as Sartre says "there's no halfway house between existing and not existing, you have to exist all the way"
so much so, the one thing descartes couldn't doubt when he did his whole "doubt everything" routine was that he existed. "I think therefore I am".
So you are alive because...
in other words, existence precedes essence - so right now literally you have to find out why you are alive.