How do you define happiness?
I suspect what most mean by happiness is desire fulfillment. You desire sex, you desire money, power, some material object. Your desire is fulfilled and that happiness is actually a temporary lack of desire.
The problem is that there is no end to desire. Once one desire is fulfilled, another soon comes along to take its place. That moment of happiness, that moment of desire fulfillment never lasts long.
So when are we actually happy? That moment when we are without a desire to fulfill.