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.
Please advise.