Trust me, real joy is a serious thing. Do you think someone can, in the charming expression, blithely dismiss death with an easy disposition? Or swing open the door to poverty, keep pleasures in check, or meditate on the endurance of suffering? The one who is comfortable with thurning these thoughts over is truly full of joy, but hardly cheerful. It's exactly such a joy that I would wish for you to possess, for it will never run dry once you've laid claim to its source.
-- Seneca, Moral Letters, 23.4
* Source: The Daily Stoic by Ryan Holiday and Stephen Hanselman