Why was the prince called a happy prince?

During his life on earth, the prince was called the Happy Prince because he was always happy.

Accordingly, the prince never knew unhappiness when he was alive, as he lived in splendor and ease. He lived in a palace where sorrow was never allowed to enter; a high wall separated his palace grounds from the outside world. Because his existence was so sheltered, the prince never knew of the misfortunes and miseries his subjects experienced in their own lives. The prince tells the swallow that his own courtiers used to call him the Happy Prince, only because he was always found to be cheerful and untroubled.

The prince remembers living a life that consisted of largely unvarying daily pleasures; during the day, he cavorted with his companions in the garden, and in the evenings, he led dances in the Great Hall of the palace. It was only when he died that he became sad, and this was because they placed his statue so high up on the city grounds that he could see the surrounding miseries and degradation in the city.