There are certain yoga laws and principles that are, shall we say, less tangible than others. For example, the law of karma. Science has proven what goes up must come down, but that's about as far as it's gone. To believe that for every action, word, and thought, there is an equal consequence takes something more intuitive, more personal; it's more metaphysical.
I think leaving [death] can be as joyous as - probably more joyous than - being born, because being born is very physically uncomfortable for the baby.