Happiness is a temporary emotional state that comes and goes. Perks make employees happy. Free catered lunches make employees happy, so does higher pay and unlimited vacation, until it does not.
Fulfillment comes from being part of something greater than ourselves. It comes from being part of a worthwhile cause, from being part of changing the world for the better. Fulfilled employees will stick around when things get tough because they believe in the cause. Everyone deep down inside seeks to be part of something bigger than themselves.
Happy employees are great but fulfilled ones are better. Happy and fulfilled teams change the world.