The happiest man on earth would be able to use the Mirror of Erised like a normal mirror, that is, he would look into it and see himself exactly as he is. Does that help?
J. K. RowlingHermione: You! You foul, loathsome, evil little cocroach! Ron: Hermione, no! He's no worth it.
J. K. RowlingThat evening, Dudley paraded around the living room for the family in his brand new uniform...As he looked at Dudley in his new knickerbockers, Uncle Vernon said gruffly that if was the proudest moment of his life. Aunt Petunia burst into tears and said she couldn't believe it was her Ickle Dudleykins, he looked so handsome and grown-up. Harry didn't trust himself to speak. He thought two of his ribs might already have cracked from trying not to laugh.
J. K. Rowling