Happiness can mean many things to people. but it is important to make one distinction: Happiness is not intoxication. I use the word intoxication very broadly, and there may be a better word for it in the English language - I am not a native speaker.
Why the word intoxication? In Buddhist theory, there are 3 "poisons" (or "toxins") for the mind - aggression, wanting and ignorance. These poisons create unhappiness.
Intoxication is everything that gives you a "high". Highs are always defined by its two phases: Coming up and coming down (as described in CPD). Intoxication is counterproductive to happiness. This applies to everything we do: Drugs and orgasmic sex are some of the easily identified ways of getting intoxicated.
One peculiar thing I have discovered, is that the three poisons of the mind can be intoxicating - to be really angry and mad at something or someone can actually create a rush, a high - and it can lead people to believe that aggression is good, because it feels good. In the long-term, aggression will, however, lead you to unhappiness.
The challenge is then to identify the parts of our lives that are toxic to happiness and avoid them. If we cultivate them, we will only cause ourselves more suffering. This is not a matter of faith - it is a matter of experience.
What happens from there - well, I am not there yet. I am still learning myself, trying to experience these theories for myself. But I feel that writing them down both helps me, and has the potential to inspire others.