I have a simple reply..........scarcity of sorrow or sadness is happiness.............
Well I agree with you that its a toughest question but people think their pleasure is happiness........but pleasure is not permanent.......but happiness can be..............One cannot be 100 % happy........because its like dark and light........there cant be 100 % dark or 100 % light............when there is a dark then the degree or percentage of light is reduced to maximum.........when light is there vice versa..........same way when the sadness decreased happiness increases....and other way happiness decreases sadness increases......
Regards,
Anil