Ego had learned very early on that he could get away with omitting certain truths when it came to interacting with other sentient beings within the galaxy. He could tell half-truths and would be able to get away with a clear conscience no matter the stakes – and there had been plenty of times where that was a necessity when attempting to placate those he'd deemed worthy of carrying his spawn. Even so, this quality had never failed him with those he truly cared about like his River Lily; like his son, the only one of his offspring whom carried the celestial gene.
But when his planet - the entirety of his existence - and his life's mission are being destroyed by that very same son, Ego does something he's never done before. He lies, begging Peter not to kill him, or else he'll be mortal once the light within Ego's core is snuffed out.
Even while Ego's physical form turns to sand in Peter's hands, Peter is still half-celestial and he never needed his father's help post-conception for it to be that way.
Yondu still sacrifices himself for Peter's sake, and Peter doesn't concentrate on bringing the light back because, why would he? He doesn't realize that there's god-like capability still within him, and if he were aware it would only sicken and further traumatize him.
Yet he still scrapes by as the leader of the Guardians of the Galaxy, still manages to outlive impossible situations none the wiser, and it's all cool.
Until Rocket dies. And Mantis follows him not long after, as does Kraglin, and then Drax, and Gamora as well – all while Peter hasn't gotten a single silver hair. He's in denial amidst all the grieving he must do and the isolation he imposes on himself because if he accepts the fact that he remains when the rest of his family are all gone, it'll drive him mad. And someone has to be there for Groot; as heartbroken and depressed as Peter is, he won't let Groot go it alone.
But Groot can't live forever. Even he, perceptively wise after living a long and happy second life, withers and falls prey to time.
There comes a time when they land on a quiet planet and settle in a meadow where daisies and dandelions grow, and they sit beneath a clear, dark blue sky where the stars are spread above them. Peter can remember sitting on the back porch of his grandparents' house, huddled close to his mother while they stargazed.
He feels like he should be prepared for death, but he isn't. He's never been. Groot smiles down at him, tranquil and sad. Groot knows that Peter won't die, that he won't get to be with their family until the universe itself no longer exists, and maybe not even then. All Groot can do to comfort his friend and father is to gently place his hands on either side of Peter's face and brush the tears away. It's a strikingly familiar gesture but Peter doesn't recall what it reminds him of, he doesn't want to remember.
Instead he tries to hum a song he hasn't heard since he was a little boy sitting forlorn in a hospital hundreds of years ago.
Groot's grip fades, and Peter is the only one staring at the stars in a field of green.