Happiness is inner joy. This is a fundamental law that we are all governed by in one way or another.
To choose to be happy, and find inner joy in the process, is a choice that you can live with, but not an easy one. It takes effort and time, and then you have to find the things that make you happy, in whatever way, and the sooner you start, the better it will work for you.
Yearning for inner joy is the yearning for your deepest yearnings and desires. That yearning is the best answer that you could ever receive from anywhere else. The deeper your yearning for inner joy, the more important it is for you to take steps to manifest it. Your yearning for inner joy will be the same as your yearning for joy.
Yearning for inner joy means yearning for your deepest yearnings and desires. It is the yearning for your most basic yearnings and desires. If you are able to recognize this yearning, you will have more than enough power over it.