An atom or molecule or a rock, considered as matter, will never die, according to the law of conservation of matter and energy. Matter can take different forms, and these forms can corrupt or morph one into another, but matter itself seems imperishable. Moreover, since a composite form can be destroyed by being taken apart, an elementary particle is forever, because it is simple. An electron cannot be disassembled.
On the other hand, human souls, as far as the naked eye can see, are eminently corruptible. A man dies, and his soul seems clearly to disappear; where it goes, no one knows. There is no “he” anymore; “he” is dead and gone.
How come matter is immortal, yet something far more noble than it, souls are not?
The solution takes into account the fact that for humans, creation is metaphysically good, morally evil, and physically ambivalent.
It is metaphysically good for all things, because everything likes its own nature; everything likes being what it is despite, unlike God, being limited. I am happy being human and want to turn neither into a frog below my level of perfection nor an angel above. Similarly, a frog does not hate its frogness, desiring instead to be a human.
It is morally evil, because without divine assistance, by using his natural powers only, man cannot become lovable enough to be dear to God or beloved by God.
It is physically ambivalent, because happiness can be achieved in this life but only in an imperfect form. Every life contains plenty of joys and sorrows.
Now man becomes lovable by being virtuous. Virtue defines one’s personality. But all 1st-grade matter has no personality. One atom of hydrogen is exactly like any other. Hence, we might say that matter is virtuous vacuously: its actual virtue equals its potential, maximum possible, virtue, except that its potential virtue is zero.
Atoms and rocks always obey natural laws, obey their nature. They never choose to do something wrong or evil. For this reason, that is, because of its fealty to God’s law, matter is granted this primitive level of eternal life.
On the other hand, man, in this world, sins. He sins occasionally even with divine assistance. He is not very lovable, at least not enough to avoid death. So, he does not deserve to be immortal in this life.