Being happy seems to be a mark of individuality. Each one of us is happy in his own way. Being happier than the other, or the happiest at one moment of time, or just simply happy for different reasons, i think is one's own sense of self preservation. You value yourself, naturally, instinctively selfish, subconsciously wanting things that would provide for your benefit because no one actually wants to be sad.
Personally though, I believe that no one can entirely be happy. Happiness since is an attitude, not a state to be achieved.