Happiness
We all desire it, and when we achieve it, we don't realize it.
I think it's because many people don't understand the meaning of happiness: many "see" only physical well-being, others the achievement of "success."
In my opinion it's a combination of well-being, meaning, relationships, and clarity about one's life. It's not a continuous state, but a quality of how one lives in moments when one feels "right" with oneself and the world. And above all, it's not found outside of oneself, but must be sought within