Living forever flies in the face of the natural order of things. Imagine the universe as a single, organic entity that is constantly creating new life while life simultaneously is ending. A cycle if you will.
Living forever devalues life because it negates purpose. Anything you can do today you can put off until next millennium as you will be alive then as well. The whole anxiety of dying is experienced only when one has not found a purposeful life. This is why Witnesses are obsessed with living forever. They have no purpose now because they believe that this life is not "the real life". For them all purpose begins only when they are on the other side of Armageddon.
With that being said, I have also recently become acquainted with the theory of the Big Bounce. That is the theory that universe has no beginning and has no end. It only resets itself every so often with a Big Bang. Then there's the First Law of Thermodynamics. If energy cannot be created or destroyed in a closed environment (e.g. the universe), it can only change forms, then what happens to consciousness when the body dies?