"Every Prime has a purpose. Primus chose them for a reason, and at the moment of the All-Maker joining their Spark they were remade for that reason."
Optimus knows this, hears it and understands it.
But he does not grasp it entirely.
The echoes of Sentinel Prime advise him, and he is well suited to give such advice.
Sentinel was chosen as Prime in a time of total war against the Quintessons; to be the rallying leader all Cybertron could follow, a bot who came from nowhere so he could be part of everywhere. Iaconians artisans stood with Kaon engineers and Simfur survivalists under the banner of the Sentinel of Cybertron.
(Even ages later, no one is really sure if Sentinel was his name for sure. Maybe just a title, hung over him to define his role. That whatever he had been, had been elft behind. Optimus tried to put the name 'Zetta' in, to give Sentinel his agency back. But history does not like its heroes mortal, and the name Zetta winds up an afternote after all.)
Optimus asks him what his own role is, and Sentinel seems awkward about it.
"You were chosen to give the planet new life," comes the eventual answer, and Optimus understands. He rededicates his efforts towards the revolution, to tear down the Functionalists and end all stasis, and begin a new transformation of the world.
The full implications of his purpose do not occur to him until the third time he should have stayed dead.
The Autobot troops begin to talk about it; it starts on the battlefield against the Functionalists, and eons later they're still talking about it when they fight the Decepticons.
Optimus has taken blows no one should survive, or even notice before dropping dead. He's been nailed point-blank in the head by railguns, powered through explosive fields of grenades and mines, been punched full on in the face by a combiner, been blasted through his body with holes showing through time and time again. There were worse injuries, too.
Sometimes, he doesn't notice the injuries until Ratchet drags him to a medic bay. Other times, he wakes up one time too many on an operating table with a medic swearing that his Spark had off-lined for a few seconds long enough to keep him dead.
The first doubts and fears creep up then. He, at least, begins to suspect.
But his role is to revive Cybertron. He focuses on that; first, to take down the Functionalists.
But then the Decepticons replace them, and tyranny is worse than profit-based oppression; consumed by rage and vengeance and the drive to never again be on the bottom, the Decepticons would ravage every planet in the universe and put all its life to the sword if that's what it took to ensure their peace. His goal now is to give Cybertron new life in a very literal sense, when all life has been bled from it by Megatron's hands.
And eons later, he can't lie about it any more as he wakes up, remembering that he was dead.
And he still stands up, alive again, and Optimus Prime now thinks about how many times he should have died.
And at last, he understands. He hasn't escaped death. There is no luck here, no survival by the epidermis of his faceplate; he has died, over and over and over again. And each time, sooner or later, he arises once more.
Optimus puts his hand over his Spark, the burn of it below his hands. So many times he felt it extinguished, and then blazing back to life again.
Sometimes he thinks he hears a voice. Many thousands of voices, millions, billions upon billions, speaking so loudly that it sounds like a titanic god-voice. Maybe just one, and the smaller voices are its offspring and choir at once. Maybe its the same thing.
'No. Not yet.'
And he awakens, alive and surrounded by the dead that he should have joined.
He is the Prime of rebirth. Revival. Resurrection.
And until his duty is complete, when at least Cybertron lives once more and all their people are at least one and truly free, and the Decepticon's legacy of terror is put to rest and all grievances, settled, he is not allowed to die.
Until its metal soil lives again, the rest and peace of a grave upon Cybertron is denied him.