Imagine if all of us are immortal. Will we really be happier? There is no rush to do what we must do. Most of us may lose motivation. Our sense to cherish others will diminish because everyone lives forever.
People who remain discipline continue to reap benefits from their improvements ad infinitum. On the other hand, people who decide to do something about their lives later continue to be outraced by the former.
Early birth gives you a better chance of outpacing others. In the long run, this creates a huge socioeconomic gap.
Being immortal takes away the meaning of our lives. It erases the urgency to bond with others, the evolutionary trait that kept our ancestors survived.