This is only a theory.
Suppose you had lost the will to live, so you decide to end your life. You purchase a gun, find the most comfortable place to sit down and pull the trigger. But to your dismay, instead of experiencing whatever existence you believe to have after death, you find yourself in terrible agony, bleeding profusely from the wound in your head. Police and medical teams arrive on the scene in a matter of minutes, you are taken to the hospital where you learn you will live to see another day. Now in a psychiatric ward, you ponder to yourself how you managed to live. Surely a shot through the temple would kill you. By some miracle, you have survived what would have ended the lives of 99.99% of others that put themselves in the same scenario.
How could this have happened?
The fact is, you ARE dead. Just not from your point of view.
If you consider that every day you are presented with choices that would ultimately change the course of your life, and that each choice branches off into an alternate universe. For example, you go to the humane society to get a pet, do you buy a dog or a cat? You believe you bought the cat however there is another version of the universe in which you bought the dog, you are just unable to experience it because of the decision you had made. Every decision you make provides an alternate "lifeline" in which you experience the universe. Free will would suggest that there are an infinite number of choices you could make, hence there would be an infinite number of universes.
Now, what are the chances that you survive a shotgun blast to the head? 1 in 10000000000000000? Even less than that? Chance is made completely irrelevant because there is an infinite number of chances that will survive. You could say that you died 4968460560937 times before you experienced the one time you somehow managed to survive.
With death, your consciousness would cease to exist in one universe and would be transferred to the one which you had survived. The people in the previous universe would perceive you to be dead, but because of your consciousness, you are only able to experience surviving the incident and life afterwards.
You are immortal, at least from your point of view.
Immortality sounds good. But consider the horrible disfigurement and agony you are now in because you managed to survive with 1/4 of your head missing. Now suppose you get into a car accident and are terribly maimed, you may die in a number of universes but ultimately you experience survival, and you keep on living, now without your right arm. Eventually you are going to get old and die a peaceful death from natural causes right? You will, but you won't get to experience it. Old age normally would be the end of you, but that heart attack or stroke that killed you 549847347 times before, didn't this time. You will live forever, becoming more and more mutilated, and because of the infinite number of branches that can be created, one leading to your survival, you will never die.
Unfortunately, if any of this is true, we are assured to suffer for eternity.
If you don't understand the theory of alternate universes etc., try reading the following:
http://en.wikipedia.org/wiki/Multiple_univers es
http://en.wikipedia.org/wiki/Many-worlds_inte rpretation