Happiness is what your feel when you feel like there is nothing missing in your life.

"Happiness is there when you remove the sense of something missing in your life. Happiness is the state when nothing is missing."

- Naval
“Comparison is the thief of joy”

If you only wished to be happy, this could be easily accomplished; but we wish to be happier than other people, and this is always difficult, for we believe others to be happier than they are.
Unhappiness comes from wanting what others have.
Happiness comes from wanting what you already have.
From Charlie Munger, on applying the mental model of inversion for happiness:

Instead of asking how do I attain happiness, how can I avoid depression.
Sleeping well, eating right, etc -- by doing that, you are 95% there to achieve happiness.
The happiness story:
  • Pleasant life: A life filled with positive emotions
  • Good life/Engaged life: A life where work and play make time stand still
  • Meaningful life: A life where a person uses their character strengths to serve a larger effort