Joy won’t lessen for the joyful (or the partier).

Harun al Rashid, an Abbasid caliph, went around at nights disguised as a poor person to see what his subjects were doing in his capital, Baghdad. In one of these night journeys he encountered a poor person who would always party and have fun even though he didn’t have much. Harun tries to make him more miserable to see whether his joy will continue. Harun rewards him in the end because the poor man continues his joyfulness even in the most trying circumstances.