You can't gauge happiness on a scale the same way you can't gauge any other emotion.
"I don't feel so happy today, I went to school and got an F in science. I would say I'm at a 3."
"Holy stuff that girl had 3 tits 10/10"
"Well, I just lost all my money because my investor decided to take a permanent vacation to the kaymen islands, I think today is the day I end it all. 0/10"