My own conviction is that once in his life Marcus Aurelius had a real good time.

He came home feeling pleased with himself without knowing quite why.
"I will write it down," he said to himself, "now, while it is fresh in my mind." It seemed to him the most wonderful thing that anybody had ever said.
Maybe he shed a tear or two, thinking of all the good he was doing, and later on went suddenly to sleep.

In the morning he had forgotten all about it, and by accident it got mixed up with the rest of the book.

That is the only explanation that seems to me possible, and it comforts me.
We are none of us philosophers all the time.
Philosophy is the science of suffering the inevitable, which most of us contrive to accomplish without the aid of philosophy.

Marcus Aurelius was an Emperor of Rome, and Diogenes was a bachelor living rent free.