Something has always existed, because by definition nothing does not exist and can't do anything. 
Anyways, lets say you have a chess board (the universe) and you know how the pieces move around (mathematical relationships). Eventually you will execute every move (as the probability of any possible event occurring reaches 1 over time). 

Eventually you'll be sitting here, reading this message board, and have no clue whether you're the first "you" or another "you". You certainly don't remember what you were before you were born, so you should probably not stress too much of your impending demise, you as a part of the universe, will be recycled. 

From star dust to Earth dust, it'll continue to happen as long as the universe continues to exist. Is the universe a constant? Do constants exist? Nobody knows... yet... 

Do you really want to live forever? Is immortality really that ideal? Well guess what we're already living forever, our molecules, the brain, just take a different form. The interesting part is that we are literally the universe. We are the way for the universe to know its very own self. 

Still don't believe me? Read into epiphenomanilism. Physical entity (molecules) to brain activity (electrical entity) to physical entity (molecules) again and again. The fractal of existence.