Happiness is not something you can directly create, it is the side effect of the things that you do. It is similar to weight, you cannot directly change your weight, but your weight changes as a result of what you do.
This makes happiness a great metric.
If you track what makes you happy and what does not, you can increase those activities that make you happy and let go of those activities that do not.
As a note, happiness is not the only metric to use in life, it is just one metric. There are many other metrics including fulfillment, legacy, health etcetera that must all be taken into account.