•Why it earns a slot
Written for an audience of one and never meant for publication, which is exactly why it still reads honestly two millennia later.
The private notebook of a Roman emperor practicing Stoicism on himself. You control your judgments, never events; obstacles are material for character; you will die, so spend attention accordingly; and other people's opinions are not your business.
This distillation is written from the freely available original, which is always the better read when you have the time: gutenberg.org.