It's not just one thing.
People who keep diaries rarely disclose what they write. Who wants to share their most deepest, vulnerable, and fleeting thoughts with the world?
The entire first book of “Meditations”, Marcus Aurelius’ personal diary, is Marcus reflecting on the people who helped him become the person he became. The diary continues with themes built upon what Marcus learned from others: death, service, justice, wisdom, nature, and acceptance of the present.
Who are the people who contributed to the you of today? What lessons, good or bad, have they taught you?