Hapiness meter unlocks around level 6 or 7. It is a happy face in the top right corner.
You fill it by clicking yellow speech bubbles that pop up on the playing area. There is a timer of 5 min between these speech bubbles. To fill the meter completely you have to be in the game actively for around 30 min. When you leave the game the meter goes down again.
The last part of the meter says "premium". This doesn't have any effect on the steam version. Jut click on those speech bubbles until the happy face reaches the very end and the achievement unlocks.