I was recently informed that it is frowned upon to keep a personal journal because:
1) There is a possibility of your sins being exposed to others
2) There is a possibility of unintentionally revealing the sins of others
3) There is a possibility that your thoughts being misinterpreted
Basically, what they say is that incase you put details of your day to day life in a journal, and if someone happens to read it, it can lead to fitna.
Hence, unless you are only you using your diary to jot down the good points, it is makruh.