Bit Moody is an exceptionally lightweight and easy to use mood journal.

Tap the bottom left and bottom right buttons to log how you feel every morning and evening respectively.
Select from 18 color-coded emotions in 6 categories.

Every time you log your mood, you can optionally include a small note.
Logs that contain a note are marked in their corner.

Reflect back on your mood throughout the previous days from the main screen.
Or place a widget on your home screen.

Your data is stored locally on your device and cannot be accessed by anyone but you.