God is what's good in me.
Johnny said this when he was six, and it says much about his life goal, which was also stated when he was young: to do some good for the world. Above his supreme intellect, Johnny has great humanity, wanting to help others as best he can. Yet this belief transcends mere altruism, and, as an agnostic (someone who does believe there is any proof of God's existence, but does not deny the possibility of said existence), Johnny wrestles with spirituality throughout his illness. He writes the "Unbeliever's Prayer" and tells God, if He exists, that he will "fight the good fight." Johnny does not need the existence of God to make him live a moral life; God is inside him, and he shares it with others.
I have so much to do! And there's so little time!
After he receives his first x-ray, Johnny makes this somewhat fearful exclamation to Frances. Beneath his outward bravery, Johnny seems aware that he will die at this early stage, and this acknowledgment makes his bravery both incredible and possible—he must accept his fate and do with his life what he can. But Johnny's fear does not center so much on himself and on his curtailed life, but, instead, on his work—what he has left "to do." He throws himself into his scientific studies with abandon, concocting experiments and even corresponding with Albert Einstein. In the doctors' condolence letters, all express regret for Johnny's unfulfilled potential. But even in his death Johnny "does" something—his miraculous fight against the tumor will be used in neurological studies.
I hope we can love Johnny more and leave behind us, as he did, the love of love, the love of life.
Rather than grieve for his death, Frances seeks a different way to remember Johnny—by thinking of his joy of life. She admits that she may cry while out on a boat, thinking of the fun Johnny would have had, but this also bolsters her appreciation of the world around her. She cannot answer any of the big questions about death, but these small joys add up to one big answer: love. She also takes comfort in knowing that Johnny has influenced not only herself, but others.
"Oh how tired I feel."
Perhaps Johnny left his diary out on occasion, as Gunther believes, to communicate to his parents indirectly, but even his complaints are far from self-pitying. If anything, "Oh how tired I feel," is more of a disappointment that he cannot do all the things he wants to do. Johnny constantly shields his parents and others from his secret pains—Gunther remarks that he pours out his secret fears to a sound recorder—and this diary entry shows the tip of the iceberg. However, if he knew his parents would read it, perhaps Johnny purposely transcribed only his semi-painful thoughts, thinking it would make them believe that, while he was in emotional pain, as any human would be, it was not an overwhelming pain that he could not handle.
Scientists will save us all.
This note is scribbled above Johnny's last letter, to Frances. It reveals Johnny's utter faith in the pure sciences as well as his hope that, one day, science will provide help in ways it could not help him. Watching medicine consistently fail to cure him must surely have been debilitating to Johnny's confidence in science, but just as he survives through courageous optimism and refuses to sink into hostility, he still believes science will someday defeat the world's ills and harbors no resentment against that which failed him.