Gratitude journaling can be a useful tool if you're depressed but it's also useful if you want to understand yourself a bit better.
Think about it: when you're down, you notice the traffic jam and not that the sun's shining, that you've had a good cup of coffee, that someone's put a newspaper out for you to read etc etc.
Keeping a note of the good stuff can therefore help you focus on the positive and that helps pull you out of the blues.
But there's another useful side to the journaling, and you don't have to keep endless notes either. Understanding exactly what makes you happy, can reveal needs and values that you may not realise you had.
For example, a few years ago I realised I was grateful when a friend called me to tell me a seminar time had changed. When I analysed it, I realised I was happy because it made me feel part of the group. Until that exercise, I hadn't realised that belonging to that group was particularly important to me.
So if you're curious about knowing yourself better, just complete this sentence: yesterday XXX happened. It made me feel great because it made me think YYY.
And enjoy the revelations!