happiness is where you find peace, everything you see would be taken into positive accounts until somebody would really spoil your feelings by telling you the truth. but for me, happiness is where you stop of being sad. everyone will die. that would make sense when you know something sad is gonna happen, and you are saddened by that. because we are always sad, even life would be tough or easy.
the true believers are always happy because they understand what their life means. well doesn't mean they can't be sad, yes indeed they are when they have done wrong but eventually they'd turn to Allah, and seek for His forgiveness.
May Allah guide us.