But who would like to live forever in this mundane world, filled with pain, decay, sorrow, and tribulation, and grow old and infirm and yet have to remain with all the vicissitudes of mortality? I think all of us would come to the conclusion, if that proposition were placed before us, that we would not like to have it. We would reject it. We would not want life of that nature. Life here in this world is short of necessity, and yet all that is required may be accomplished, but death is just as important in the plan of salvation as birth is. We have to die—it is essential—and death comes into the world ‘to fulfil the merciful plan of the great Creator.’
(Joseph Fielding Smith, Doctrines of Salvation, 1:116)