In Phaedo to argue the soul exists after death with the cyclical argument, Plato posited a philosophical assumption that all opposites —e.g. less, greater; weaker, stronger; sleeping, waking; life, death—are generated out of each other.
One argument for this assumption offered by Socrates is in the quoted text. It tries to prove life is generated from death, just as waking from sleeping. It states if life goes into death but death never back into life, then there will be no life and all things will remain in the state of death.
Is the assumption that all opposites are generated out of each other reasonable?
And that these admissions were not unfair, Cebes, he said, may be shown, I think, as follows: If generation were in a straight line only, and there were no compensation or circle in nature, no turn or return of elements into their opposites, then you know that all things would at last have the same form and pass into the same state, and there would be no more generation of them.
What do you mean? he said.
A simple thing enough, which I will illustrate by the case of sleep, he replied. You know that if there were no alternation of sleeping and waking, the tale of the sleeping Endymion would in the end have no meaning, because all other things would be asleep too, and he would not be distinguishable from the rest. Or if there were composition only, and no division of substances, then the chaos of Anaxagoras would come again. And in like manner, my dear Cebes, if all things which partook of life were to die, and after they were dead remained in the form of death, and did not come to life again, all would at last die, and nothing would be alive—what other result could there be? For if the living spring from any other things, and they too die, must not all things at last be swallowed up in death?
-- Phaedo 72
BTW, a quite similar question is asked on this site about why the cyclical argument fails, but it didn't consider the above quoted argument, so I don't think it's a duplication here.