Happiness is an experience.
What controls your experience?
Your attention: what you focus your attention on, you experience.
What controls your attention?
Your choice: only your choice can change what your attention gets focused on.
What controls your choice?
Nothing: you ARE choice; you are the freedom to choose.
All we are is the freedom to choose what we focus our attention on, and thus everything we experience.
Remembering that we have and are this choice is happiness.
Forgetting that we have and are this choice is unhappiness.