Throughout his life, Sam had hated being told to 'fight,' as if sickness were a character failing. Illness could not be defeated, no matter how hard … - Gabrielle Zevin

Owen doesn't watch when they place Liz in the River, next to all the other babies who would be born that day. Nor does he watch when the launch nurse pushes Liz away from the shore into the current that leads back to Earth. To the untrained observer, it seems as if Liz's departure has no effect on Owen whatsoever.
Curtis Jest watches Owen before deciding to go over to him.
"Owen," Curtis asks, "do you remember who that was?"
Owen looks up from playing with the boat. He appears to find Curtis's question difficult. "Lizzie?"
"Yes," says Curtis, "that was Lizzie. She was my friend. She was your...your friend, too."
Owen continues playing with the boat. He begins singing Liz's name in the unaffected way children will sometimes sing a name. "Lizzie, Lizzie, Lizzie," he sings. Owen stops singing abruptly and looks up at Curtis. A horrified expression crosses Owen's face. "Is she...gone?"
"Yes," says Curtis.
Owen nods. "Gonegonegonegonegone." Owen begins to cry in an undignified manner, although he isn't entirely sure why he is crying. Curtis takes Owen's hand, leading Owen away from the puddle.
"You know," says Curtis, "you may see her again someday."
"Cool," says Owen, and with that, he stops crying.

A life isn't measured in hours and minutes. It's the quality, not the length.

It's when you don't need something that you tend to lose it.