When the Serpent assures Eve in the first part of Back to Methuselah that "Death is not an unhappy thing when you have learnt how to conquer it," we are puzzled by the implications these words have. For Shaw, at least at that time, death is nothing but a necessary incident in evolution that allows species to survive by avoiding overpopulation. However, that does not mean that we cannot conquer death. As the Serpent puts it,
THE SERPENT. The serpent never dies. Some day you shall see me come out of this beautiful skin, a new snake with a new and lovelier skin. That is birth.
But the words that triggered this post are to be found in the Preface to Misalliance ("A Treatise on Parents and Children"). Shaw's train of thought to arrive at this conclusion is quite straightforward: If you follow the teachings of Judeochristianity, "if you wish to live for ever you must be wicked enough to be irretrievably damned, since the saved are no longer what they were, and in hell alone do people retain their sinful nature: that is to say, their individuality."
That is why "Death is for many of us the gate of hell; but we are inside on the way out, not outside on the way in. Therefore let us give up telling one another idle stories, and rejoice in death as we rejoice in birth; for without death we cannot be born again; and the man who does not wish to be born again and born better is fit only to represent the City of London in Parliament, or perhaps the university of Oxford."
I particularly enjoy the last bit, don't you?