Because we were sent to the world by the God to do homage to Him; our life is not ours but belong to the Almighty; If Allah has called us to set back to Him, no one can obstruct it. If you say, we (in general) do not know the time when we will pass away -- but this condition is not for all human. Many muslims have known the time when they will pass away several days before their death; a couple of weeks before they die; a couple of months; and one year before their death. It depend on their belief quality; those ones who get the signs of their death a year before the Angel Izrail (the angel of death) comes to pull out their soul, usually have belief quality over the average; so with the months, weeks, or days. As far as I know, the death sign will come to every body started from 1 year, 6 months, 3 months, 40 days, 7 days, and 3 days before the death -- no matter what religion they are in. The problem is, many of the concerned humans do not care with the sign -- and for some others who can catch the sign remind calm and try not to tell other -- afraid of their sadness. But some wise men among muslims that I ever know in this matter, usually told the signs to the ones that they really know well.