I think part of the problem here, and why writing a qualitative description is a good idea, is that forcing yourself to "quantify" your happiness (an incredibly difficult thing to do even in the best of circumstances) is going to inherently lead to inaccuracies. I'd even consider making the argument that having to quantify your happiness could inadvertently lead to you actually being -less- happy. Imagine it - you feel fine one moment, then, as a result of a ping, you now have to evaluate exactly how you're feeling and your day so far and turn it into a number. What if that number doesn't come out the way you want it to? Now you're unhappy.

That's a lot harder to do with a qualitative observation, where you can quickly spitball how you feel at the immediate moment without in-depth contemplation.