Happiness is contagious - the recently released the results of a 20-year study showing the “dynamic spread of happiness in a large social network", suggest that happiness, like health, is a collective phenomenon.
Money can buy you happiness, after all, according to authoritative recent research - but only so long as you spend the money on someone else.
Ignoring outside pressures, an increasing number of people are simply "choosing" to be happy, to follow an "optimism manifesto".