Why was the prince called the Happy Prince?
The statue of the Happy Prince was called so because when he was alive, he lived in a palace where sorrow was not allowed to enter, so he did not know what sadness was.
Here is a passage from the story, The Happy Prince, by Oscar Wilde:
“In the daytime I danced with Beauty in the Garden, and in the evening I led the dance in the Great Hall. Round the Garden ran a very lofty wall, but I never cared to ask what lay beyond it, everything about me was so beautiful. My courtiers called me the Happy Prince, and happy indeed I was, if Pleasure be happiness. So I lived, and so I died. And now that I am dead they have set me up here so high that I can see all the ugliness and all the misery of my city, and though my heart is made of lead yet I cannot choose but weep.”
Now that he is a statue, he can see all the suffering in the city, and he is no longer happy.