Why did a Roman emperor keep a private notebook he never meant anyone to read?

Because it was a tool for working on himself, not a book. Marcus Aurelius was Roman emperor from 161 to 180 and a Stoic philosopher, and the Meditations are a collection of personal writings: private notes to himself, with his reflections on Stoic philosophy. He wrote the twelve books in Greek as a source for his own guidance and self-improvement, and it is unlikely he ever meant them to be published. The work has no official title; "Meditations" is just one of several names people have used.

The emperor wrote some of it while on campaign. Internal notes say the first book was written in the country of the Quadi, at the Granova, which is the modern Hron River in Slovakia, and the second at Carnuntum. Those were hard years: the Antonine Plague, which broke out in 165 or 166, is estimated to have killed five to ten million people across the empire. The notebook offers a window on his inner life, though it is largely undateable and says little about worldly affairs.

What did he tell himself? One central theme is analysing his own judgments and those he makes of others, while keeping a cosmic perspective. He reminds himself that he has the power to strip away many superfluous troubles that lie wholly in his judgment. When someone speaks badly of you, he notes, that is the report; the report was not that you have been harmed. And he promises to change gladly if anyone proves he has erred, since he seeks the truth, by which no one has been harmed.

The book's survival was a close call. Its first clear mention, from around 907 AD, comes from a bishop and manuscript collector, Arethas of Caesarea, who wrote that his old copy was altogether falling to pieces. Today's text rests almost entirely on two manuscripts, and one of them is now lost.