Why the boy happiest in world title?

It seems you're asking about the title "The Boy Happiest in the World". This isn't a widely known or recognized title, so it's hard to give you a definitive answer. However, I can speculate on a few possibilities:

* It could be the title of a book or movie: There may be a story about a boy who is exceptionally happy, and this is the title used to describe him.

* It could be a phrase used in a song or poem: It's possible that the phrase is part of a creative work, expressing the idea of a child's simple joy.

* It could be a personal expression: Someone might use this phrase to describe a specific boy they know, highlighting his happiness.

To give you a more helpful answer, could you please provide some context?

For example:

* Where did you hear or see this title?

* What is the overall context of the situation where it was used?

* Do you have any additional information about the title?

This information will help me understand your question better and provide a more accurate answer.