In the bible, after the fall of man, Adam and Eve were driven out of the garden of Eden. In the garden of Eden was the tree of life, and I think God didn't want man to live forever with sinfulness and how bad it was going to get. Adam had no idea how bad earth was going to become.

Also, in the bible, the age of man is seen to reduce drastically after the flood. It has been said that it was after the flood that flesh became a main diet, as most of the plants were dead.

I don't want to live forever, not in such a world filled with pain, abuse and deep wickedness. I just hope to die when I am at peace with God and man, and when I have fulfilled my purpose.