The breakup of Alexander's empire
After Alexander's death (323 BC), his empire falls apart. The world is unstable. People search for a way to keep their sanity.
The philosophical cause
Epicurus — Epicurus: happiness lies in small things — in friendship, in withdrawal from politics. The Stoics: happiness lies in what does not depend on circumstances.
Stoicism will reach Rome, reach Marcus Aurelius — and reach our own time through Seneca.