Psychologists seem to have a very narrow-minded definition of happiness. Most of the time, they equate happiness with joy.
To probe the effect of happiness on selfishness, Forgas and his colleague Hui Bing Tan put 45 students into good or bad moods by giving them positive or negative feedback on a "cognitive test" that they had taken.When students received positive feedback, what they actually felt is contentment, or even joy, but not happiness. Happiness is much deeper than either contentment or joy. It is a lasting moment when one feels at peace with the rest of the world, where desires have finally shut up for just a moment and what really matters is the present. It is first and foremost a deep, lasting emotion as well as mental state which doesn't either come or go in a predictable way. As an example, imagine the what the wise grandfather at the family Christmas table would feel to see his offspring all grown up.
My own theory, which is obviously inspired from what I've read, is that happiness most often than not comes from a steady increase in power over oneself, a form of self-control.