people cannot live forever because of health, and stuff... If we were able to live forever we would not have sicknesses.. We eventually age and weaken and by the time your 80-90 you start to get little to major health problems and you cannot do everything on your own forever. It is sad that people die, good people and people we love but we all have to die for a reason, reasons we may never be able to explain, other than health problems or depression..
Because at some point our organs get tired and weak from all the years of pumping and circulation etc. so at some point our bodies just can't do that forever...